I'm fairly certain the author planted that little idea as bait, so they could surprise even the foreshadowing-savvy people with the result.
I don't know that it was a surprise. This is fiction after all, we kind of know the rules of multiple ending possibilities based on the evidence. So even if an end is not the one thinks should happen or will happen, if the author did their work right, we'll still be aware of the other possibilities.
The way I look at it solitar is right, it's convergent evolution in the sense that demons and humans are operating on really different frameworks of biology, morality,and culture but ended up looking similar.
What she's not right about is that means there never can be understanding at some point, since that deems she knows where evolution is going.
But that's a a black blank canvas, no one actually knows how the future will go, much less as individuals in the midst of it, we can't realistically predict the future of our own species. Not without guarantees of powers of influence and control on the world outstripping our own lifespan by magnitudes larger. i.e. I'm talking geological ages, thats the time frame evolution works, which imo outstrips even the lifespans of the elves and the demons in the story.
Afterall soltair doesn't know the killer whales and the shark did have a common ancestor 500 millions years ago. If demons survive humans in the next 1000 or 10000 years, she's not going to know how they'll be. No one does but the author in this case.
Tldr: I think gold man demon did learn something, and from the framework of evolution on eons perspective, coexistence is not impossible, it just depends on whether the demons survive to that point or if humans exterminate them beforehand. Elves though for sure will not be in the picture (fron the MYA standpoint) since they're already dying out. Doesn't mean though we can't enjoy Frieren in between.