In fairness it is a particularly ridiculous claim. It'd be crazy even without the atemporal creation, but the long history of the place really doesn't help.
We could try to make an argument about the number of atemporal spirits our there, and how giving a place a history is about as hard as giving it width if you're already engineering a new reality out of nothing and Bob's whole thing about Molly being a Queen without a court rather than one separated from hers, but I think that's a losing position.
Too much rests on what could be and not enough on what can be proven.
Even with a sorcerer taking about the nature of the realm we're still talking about abstract enough points that there's lots of room for disbelief.
The biggest points we can prove here that contradict his position are that the material here is physically real and can be taken back to earth and that the FFCF are inside Molly's soul instead of being in the nevernever.
Lord of the land is only helpful here if watching it in action gives enough information to support the second point. If it doesn't then all it proves is that we have dominion, which you can have over normal pocket realms. Mab, for example, can twist faerie in all sorts of ways but sure as hell didn't make the place.
Harry should actually be able to test and confirm the location of the FFCF himself. He knows a spell for entering the nevernever, it isn't his focus, but he does know it.
Wizardry isn't an X-men style mutant power, to do stuff like that Harry has to understand what he's trying at a rudimentary level and be able to sense what he's doing. They make a point about it in the early DF when discussing how a wizard is different than a talent.
If we can get him to take a poke at the border between this place and reality then he should be able to at minimum tell this place is not hanging around on the other side of the gauntlet he's familiar with. If he's good enough to get a "vector" when reaching to interact with the borders of his local reality and trying to connect it to somewhere else then that would pretty solidly support our case.
It being inside and composed of Molly's soul requires that she either be telling the truth or have ripped a planet out of the spirit world and made it into physical reality to support the illusion she is. Whichever he believes they're functionally the same going forward.
It might not be the most important thing, but there's still cause to do it.
Even setting aside the fact that he's one of Molly's intimacies, Dresden is a decently powerful wizard and would absolutely interfere to protect Molly. We don't want him to try that against our own hell realm.
He's also a very influential and high profile member of the council. He does have a very turbulent relationship with the organization as a whole, but that doesn't diminish the fact that he has a mix of high leadership connections and serious credibility with groups like the rising generation of the council's military arm.
They don't trust him on a lot of things, but he does still have their attention and respect on others. With his connection to Molly it's highly likely he will have great influence over how they initially see us, which in turn will influence our relations with most of the human friendly powers.
It's unlikely to be the dominant factor at that level, but it should still be enough to greatly help or hinder us depending on how it goes.
At minimum it's worth our time to make sure he won't cause us well intended problems as he runs around trying to get a handle on the situation.
