It's a little more complicated than that & a big chunk of your confusion is almost certainly due to the frankly bizarre misinterpretations & misrepresentations of the OP in this thread.I’m sorry, again I’m not versed in the setting, but does the concept of gods exist in Eberron? If it does, then I would think that the question would arise with some regularity as to whether such beings as gods actually exist. I’m under the impression that Eberron is supposed to be a setting in which that question is open-ended. If the setting book answers that question for you, however, then it isn’t open-ended any more, which negates that part of the setting.
From what I’m reading here, it seems that according to this book, it’s still true that the gods of Eberron might not exist, but it is now also true in Eberron that gods do objectively exist.
The "concept" of gods, but their existence (or lack of)is a philosophical question rather than an objective provable fact. The exception to this is The Silver Flame/kalok shah, but it has nothing in common with what any setting (including eberron itself) would call a god, The silver flame is more of a force or conflagration of mortal souls/energy of celestial beings akin to the sacred flame cantrip scaled past epic eldritch machine power levels. You can go to the flamekeep (Think the CoSF Vatican analog) to see a piece of it if you want.
Any "evidence" that gods exist in eberron is subjective interpretations & a lot like what certain extremely devout religions people on earth will point at as "evidence" god exists. If you were to cast a spell like augury/gate/planar binding/etc not only would the celestial involved be unable to give any better evidence; but they might not even want to be wasting time interacting with you instead of whatever more important things they could be doing.
Demon Overlords exist & are bound in Khyber, that is a knowable fact. Back in 3.5 they had deific ranks, but mostly that was because 3.5 had mechanics that were tied to those. There's 30 or so overlords & most are left to the gm to define (ie not even a name). The ones that do exist are often forces of nature & concepts given flesh & free will which makes them absurdly powerful should they ever be unbound.... How do you fight the rage of winter, unknowable truths, secrets whispered from shadows, etc when given a vessel of flesh & free will?.. Their existance is extremely important to eberron, but not because they are actively doing anything. They set the foundation for soooo many things from 998yk/present day all the way back to the equivalent of eberron's big bang.