They clarify this slightly elsewhere by using the phrasing of "contribute back to the open source community". You interpreted "contribute" as "contribute to th

They clarify this slightly elsewhere by using the phrasing of "contribute back to the open source community". You interpreted "contribute" as "contribute to the upstream project". Which yes, of course, no GPL license requires that. However, all GPL licenses require you to contribute code changes back to your users. That is still contributing, even if you personally happen to associate "contribute" more strongly with "upstream". I think the author meant contribute in that sense, contribute source code to the ecosystem of users receiving the changed software, which in most cases is the entire software ecosystem (since in many cases, webpages and software are offered to almost anyone in the world).