Both, yes. Though for my part, I’ll say I’ve previously underestimated the effect of phycological / hearing adjustment. Over the years now I’ve had two of the same item (one old, one new) several times, which sometimes shows perceived "large" burn-in changes as illusory. Of course this is greatly complicated by the fact that some manufacturers DON’T produce the exact same quality item from unit to unit - even when they’re supposed to be the same version. And then there are products where the maker silently works in minor changes over time (no explicit version change). And then some products - like headphone pads - really do change their acoustics from wear & conditioning.
So I’ll just be satisfied with the fact that BOTH apply, but that actual large changes from burn-in are exceedingly rare.
I once had a guy install new output caps in a custom tube headphone amp. When I listened, it sounded like complete sh*t with no bass and most of the midrange missing. He said: `don’t worry, these caps just take a long time to break in! Get a few hundred hours on them.` Well I got fed up with that quickly, because it was truly unlistenable. So I cracked the amp open and see that he’d put the 0.47uF "bypass" cap in SERIES with the main cap. So even with 300 ohm headphones the high-pass rolloff effect was starting at something like 1 kHz. That’s my best example of "burn in mythos run wild".