I'm always a little skeptical about statements like that. They also say that small speakers, as they are fashionable right now, can't sound "right" because of the too small baffle.
In-wall speakers show almost no edge diffraction (since there is an "infinite baffle" for a large frequency range), so they should actually sound particularly good or not?
The LXmini has no usual baffle, but reflective surfaces (woofer, broadband driver mount) and the entire broadband driver edge as a secondary sound source. The sound radiated to the rear also arrives directly with a time delay and lowered SPL at the listening position (i.e. "bad early reflections") and indirectly via reflections.
The following sounds a bit like a pedantic asshole, but I hope the difference in my reasoning becomes clear.
Therefore, for me personally, the simplest explanation for the sound is the radiation pattern of the speaker. This is of course a result of the "unusual baffle" (almost none), but another speaker (for example with a normal baffle, a small horn and a backward radiating tweeter) that shows a comparable radiation should have very similar sound characteristics.
In a nutshell: It's the radiation pattern, not the baffle