I suspect that 400,000 was meant as “around,” not “exactly” that time period. If you read the article which you can click to where it says Read more ....., then you will have most of your questions answered. Paleontologists are thoroughly trained to differentiate teeth from various animals and species of hominids. If you are really curious you could Google the names of the authors or researchers and find where they have published. Then you could go to a library or Google the published article and get the minute details.
The issue is the dating, how it was arrived at, and why is it valid. No one wants to answer those questions. That is the issue, not insulting everyone they do not know “has never cracked a book.” What a joke. These guys say this, post silly vidios, and expect to be taken seriously? Address the issue, don’t waste your time evading it.