so evolution has had free reign to use language sophistication as a proxy for humanity and intelligence

my point is, evolution doesn't need to be involved in this paradigm. it could just be something children learn - this thing talks and is therefore more interactive than this other thing that doesn't talk.

Additionally, at the time in pre-history when assessing the intelligence of something could determine your life or death and thereby ability to reproduce, language may not have been a great indicator of intelligence. For example, if you encounter a band of whatever hominid encroaching on your territory, there may not be a lot of talking. You would know they were intelligent because they might have clothing or tools, but it's likely nothing would be said before the spears started to be thrown.