Some of the missing are left out languages. E.g. there are ~200 million speakers of turkic languages, but they only cited Turkish with ~71 million. Amd they didn't include a single Bantu language. There must be more than 300 million Bantu speakers.

It is also kind of weird that they give numbers to the tenth part while using wild estimates. Turkey has 85 million inhabitants and up to 10 million native speakers outside of turkey. There are no official ethnicity numbers from turkey as ethnicity is not registered. Also no one knows how many turkish speakers exactly live outside of turkey. But they give us numbers to the tenth part? The situation will be similar for the other languages.