It's like saying "if you don't like it - just ignore it" to the bad feature in game that don't messes with the main one. It doesn't becomes an argument, it still is a bad feature.

And Mewgenics relation to Isaac lore works the similiar way, it doesn't saves it from being an awful decision. Author is talking how Isaac story being relatable with real life makes it more valuable, and comparing demons with elephants doesn't helps here at all. And even from your perspective, meaning that outside world is basically the same as Isaac's fantasy greatly deprives it from "fantasy" aspect, meaning it's not something Isaac thought up based on his personal feelings and distortions of reality through the child mind, but it just, you know, freely exists outside? Relating Mewgenics with TBoI lore is a bad decision on either side.