While I can see how the interpretation you refer to might come about, as far as my understanding of Nietzsche goes I would not say that he holds that joy and regret are indistinguishable. On the contrary, a goal to strive for in a fulfilled life is a radical rejection of regret. That, in fact, is the driving force behind the formulation of the doctrine of eternal recurrence: if everything that happens will repeat itself forever, we might as well choose to prospectively embrace whatever fate throws upon us (an attitude Nietzsche sometimes refer to as amor fati) instead of sticking ourselves to what might have been. A beautiful formulation of that idea, though not yet explicitly in terms of the doctrine of eternal recurrence, is Beyond Good and Evil §56:
Anyone who, like me, has, with some enigmatic desire or other, made an effort for a long time to think profoundly about pessimism and to rescue it from the half-Christian, half-German restrictions and simple-mindedness with which it has most recently appeared in this century, that is, in the form of Schopenhauer's philosophy; anyone who really has, with an Asian and super-Asiatic eye, looked into and down on the most world-denying of all possible ways of thinking - beyond good and evil and no longer as Buddha and Schopenhauer do, under the spell and delusion of morality - such a man has perhaps in the process, without really wanting to do so, opened his eyes for the reverse morality: for the ideal of the most high-spirited, most lively, and most world-affirming human being, who has not only learned to come to terms with and accept what was and is but wants to have what was and is come back for all eternity, calling out insatiably da capo [from the beginning] , not only to himself but to the entire play and spectacle, and not only to a spectacle but basically to the man who needs this particular spectacle and who makes the spectacle necessary, because over and over again he needs himself - and makes himself necessary. How's that? Wouldn't this be circulus vitiosus deus [god as a vicious circle]?
Loving fate in this manner, of course, requires acceptance of both joyful and sad occasions on the course of a life. That is related to an idea which, through not equivalent, is perhaps closer to what you stated in the question: joy and sadness are inextricably linked, and it is pointless to hope for a life filled with happiness alone. One place where Nietzsche comments on that is The Gay Science §338:
The entire economy of my soul and its adjustment by "misfortune," the uprising of new sources and needs, the closing up of old wounds, the repudiation of whole periods of the past - none of these things which may be connected with misfortune preoccupy the dear sympathiser. He wishes to succour, and does not reflect that there is a personal necessity for misfortune; that terror, want, impoverishment, midnight watches, adventures, hazards and mistakes are as necessary to me and to you as their opposites, yea, that, to speak mystically, the path to one's own heaven always leads through the voluptuousness of one's own hell. No, he knows nothing thereof. The "religion of compassion" (or "the heart") bids him help, and he thinks he has helped best when he has helped most speedily! If you adherents of this religion actually have the same sentiments towards yourselves which you have towards your fellows, if you are unwilling to endure your own suffering even for an hour, and continually forestall all possible misfortune, if you regard suffering and pain generally as evil, as detestable, as deserving of annihilation, and as blots on existence, well, you have then, besides your religion of compassion, yet another religion in your heart (and this is perhaps the mother of the former) - the religion of smug ease. Ah, how little you know of the happiness of man, you comfortable and good-natured ones! - for happiness and misfortune are brother and sister, and twins, who grow tall together, or, as with you, remain small together!
