No, what this check means is that the recently introduced multiprocess-features of Firefox won't be blocked by Tab Mix Plus. You'll get much better performance, if you have no extensions which block that.
The guy seems to be confusing things here, too.
But while this doesn't confirm it, it neither confirms the opposite. So, it is very much still possible thst Tab Mix Plus gets ported to WebExtensions before the old add-ons get shut off, i.e. you might very well be able to use it still then, there's just currently no way to know yet.
