I think the OP refers to Joseph Margolis's essay The Limits of Metaphysics and the Limits of Certainty in the Antifoundationalism volume that gives an unorthodox interpretation of apeiron explicitly inspired by Nietzsche and Heidegger. Heidegger wrote four essays on pre-Socratic philosophy, one of which is entirely devoted to his (very speculative and poetic) interpretation of the very Anaximander's fragment quoted in the OP.
On Margolis's elaboration of Heidegger, the apeiron is not so much limitless (in a physical sense) as inexplicable, devoid of articulation, the mother womb of being, from which everything discrete, expressible and delimited comes, and to which it must return as a "penalty" for its "transgression". It is not a foundation (arche), an "invariant structure" that both underlies the reality ontologically and makes it intelligible epistemologically, the idea that dominated both pre-Socratic and classical Greek philosophy. It is rather a "conceptual closure ranging over all that is real, over all that is thought to be real, over all (however unfathomed) that is possible to be
real and possible to be thought to be real - but only in a
way that cannot be formalized".
This is reminiscent of Wittgenstein's conception of that which is beyond language that marks "the limits of my world", and can only be "shown but not said". And such interpretation is at odds with the standard one, see e.g. Tlumak's review, where apeiron is a peculiar proto-arche, the ultimate invariant that underlies the more traditional elements, as Margolis himself acknowledges. But it is in line with his own pragmatist and antifoundationalist philosophical leanings (as can be said, in a more diffused sense, also of Heidegger's).
Here is Margolis on why his interpretation marks Anaximander as a philosophical enfant terrible, even more so than Protagoras, the notorious father of relativism:
"The reason Anaximander's remark is so important is that, of the
entire pre-Socratic corpus, it is the only one, faltering though it
may be, that challenges (or may be made to challenge) the canon of
Greek philosophy "from the outside." The canon, the archic canon
as it may be named, threads its inexorable way from Parmenides
through Plato and Aristotle down to our own day. It maintains that whatever is real is unchanging and is intelligible in virtue of and to the extent only of possessing invariant structures. The canon was reflectively challenged "from the inside," in ancient times, by Protagoras.' But that challenge was also nearly suppressed or discounted
by ridicule. Anaximander challenges the canon (or may
be interpreted to challenge it) "from the outside," so to say - in
the sense that what he says goes beyond the admitted resources
of enunciative discourse.
[...] Anaximander straddles two sorts of conceptual innovation: one,
that of the cosmologist, proposing an "external" source for whatever
is determinate in accord with the archai"internal" to or regulative of the familiar observable world and for those archai themselves a source (the Apeiron) possessing its own (unknown) archai utterly different from what governs what has been "separated" from it; and the other, that of a distinct, still uncatalogued sort of philosopher who realized that any Apeiron thus characterized invites an infinite regress of similar speculations confined only by our ignorance of what lies beyond the archai of the (or any such) "separated" world".
...It is also difficult to believe that a mind apt enough to grasp the "philosopher's" theme implicated in the "cosmologist's" would not have been tempted at all to construe the Apeiron (once formulated in the first sense) also as an inarticulable source, a surd, that could not be captured by any archai at all. This is, of course, related to the point of the natural (somewhat Orphic) speculation of Nietzsche and Heidegger (and of others):" and it is clearly opposed to the archic limitation
of Plato's and Aristotle's thought."
