To answer the title: the monoculture problem is not exclusive to Tor Browser. I don’t think the passage necessarily implies that it’s exclusive to Tor Browser but maybe it could be made more clear?
The way I see it it’s more like, if you want to browse on Tor and retain some level of anonymity you need the Tor Browser, so an attacker targeting Tor users will basically only have to worry about vulnerabilities in the Tor Browser. And we’re assuming the reason the attacker is attacking you is to deanonymize you, which is something you don’t worry about on other browsers because you don’t expect anonymity anyway.