From the outset, it's important to note the historical context of this quotation. It's virtually guaranteed that by "physics", Nietzsche is referring here to Aristotle's Physics, i.e. the study of all observable natural things. He's definitely not talking about theoretical physics as we know it today.
I also think it's instructive to read this passage alongside his prior work, The Gay Science. In particular, section 335, which makes clear that his defense of physics (i.e., the empirical realm) is a result of this larger aim of discrediting moral justifications:
Long live physics! — So, how many people know how to observe? And of these few, how many observe themselves? 'Everyone is farthest from himself'—every person who is expert at scrutinizing the inner life of others knows this to his own chagrin; and the saying, 'Know thyself', addressed to human beings by a god, is near malicious. That self-observation is in such a bad state, however, is most clearly confirmed by the way in which nearly everyone speaks of the nature of a moral act—that quick, willing, convinced, talkative manner, with its look, its smile, its obliging eagerness! […] So: when man judges "that is right" and infers "hence it must come about!" and then does what he thus has recognized to be right and described as necessary—then the nature of his act is moral! But, my friend, you are speaking of three acts instead of one: even the judgment 'that is right', for example, is an act. Wouldn't it be possible for a person to make a judgment in a way that would be moral or immoral? Why do you take this and specifically this to be right? […] For all that, the firmness of your moral judgment could be evidence of your personal wretchedness, of lack of a personality; your 'moral strength' might have its source in your stubbornness—or in your inability to envisage new ideals. And briefly, had you reflected more subtly, observed better, and studied more, you would never continue to call this 'duty' of yours and this 'conscience' of yours duty and conscience. Your insight into how such things as moral judgments could ever have come into existence would spoil these emotional words for you, as other emotional words, for example, 'sin', 'salvation of the soul', and 'redemption' have been spoiled for you. […] Let us therefore limit ourselves to the purification of our opinions and value judgments and to the creation of tables of what is good that are new and all our own: let us stop brooding over the 'moral value of our actions'! Yes, my friends, it is time to feel nauseous about some people's moral chatter about others. Sitting in moral judgment should offend our taste. Let us leave such chatter and such bad taste to those who have nothing to do but drag the past a few steps further through time and who never live in the present—that is, to many, the great majority! We, however, want to become who we are—human beings who are new, unique, incomparable, who give themselves laws, who create themselves! To that end we must become the best students and discoverers of everything lawful and necessary in the world: we must become physicists in order to be the creators in this sense—while hitherto all valuations and ideals have been built on ignorance of physics or in contradiction to it. So, long live physics! And even more long live what compels us to it—our honesty!
– Friedrich Nietzsche, The Gay Science, Cambridge University Press: 2001, pp. 187–189
So in the passage you quote, Nietzsche appears to be praising empirical observation of the world as a superior form of inquiry compared to Platonic idealism. In fact, recall his assertion that the "will to power" ethic permeates nature, a fact that he arrived at by observation and perspective, just like the physicists analyze the world.
But it's more complicated than that. Nietzsche doesn't consider himself to be a physicist—far from it. That's what the initial phrase regarding the "five or six minds" is referencing. He's basically saying that, of all my readers, it will probably occur to a small number of them that neither the domain of physics or philosophy is immune from turning into "an interpretation and exegesis of the world". Just like idealism and philosophy, physics is ill-suited to function as a grounded domain of inquiry because it, too, is vulnerable to regressing into mere interpretation and polemic. Even physics is all perspective, "and not a world-explanation". He wants nothing at all to do with positivism. In fact, he's rather dismissive of what he perceives as the arrogance of natural scientists in assuming that they have the answers to the world.
Ultimately, this is the nihilism for which Nietzsche is famous. If even at the fundamental nature of observation (physics), we're deluded by interpretations and descriptions of the world, as opposed to causal explanations of the events and phenomena that occur, we will be unable to separate ourselves from our description or interpretation of these events. As such, "truth" becomes simply
A mobile army of metaphors, metonyms, and anthropomorphisms—in short, a sum of human relations which have been enhanced, transposed, and embellished poetically and rhetorically, and which after long use seem firm, canonical, and obligatory to a people: truths are illusions about which one has forgotten that this is what they are; metaphors which are worn out and without sensuous power; coins which have lost their pictures and now matter only as metal, no longer as coins.
– Friedrich Nietzsche, "On Truth and Lies in a Nonmoral Sense", 1873
(from Philosophy and Truth, ed. Daniel Breazeale, Humanities Press International: 1999, p 84)