While I won't disagree that there might be other labels that perform better than "Submit", calling "submit" as the worst possible copy seems overtly sensational. We both know it isn't.
He's likely right, it's a best practice that may be wrong in some cases. Though generally, you're better off going with a best practice and then A/B testing alternatives.
The reality is that you're limited in the number of A/B tests you can do, both by traffic but also by your own time and resources. So you can run a test against something that is generally accepted (like what text works on a button) or against more important changes like headline copy or page layout.
What makes you think it's a best practice? It may be usual, but when it comes to getting people to notice (and therefore, do) things on the web, "usual" isn't usually that good.
The rest of your comment is a straw man fallacy. Of course you can't test everything. But, you know...it might just be reasonable to test the conversion button on your landing page, especially when someone who does a lot of testing tells you that it has a big impact. Because that's a more informative prior than "we should just use the default behavior".
Aren't you being too literal here? Vast majority of internet users have been trained to see "submit" for "okay I'm done filling out this form, next!".