I'm going by the (admittedly, introductory) education I had in linguistics, so if you'd like to convince me otherwise I suppose I would need to see some sort of reliable source.
If a language avoids relying on tonal information for word definition, for instance, it is very counterintuitive to say that a toddler learning would pick it up exactly as fast as a toddler that only needs to handle syllable identification.
Way more languages, including Asian ones like Japanese and Korean, do not have words which completely change definition based on pitch. I don't have any reason to think a Korean adult would find it any easier than me to detect tones in Mandarin.
I don't know how to search for a reliable source in the field of linguistics, sorry. The idea is really counterintuitive for me though. It's not like Chinese and Japanese are very similar, even if they're more likely to broadly share word roots. Assuming we aren't using pictograph writing anyway, they do share a lot of those.
But if you're going to argue that learning pictograph writing is exactly as easy as learning character writing, I'll direct you to how Japanese kids only can read 1000 characters by the time they're 12. With syllabic writing you can read anything way sooner. So clearly for written language some are easier than others too.
That's just wrong. All that means is that you haven't experienced all the crazy language features out there. Children all over the world learn their native languages at different speeds because some languages are harder than others.
I guess your statement is technically correct in that if you already know a really difficult language, learning it is easy, but that's just pedantry.