Yeah, it is funny to read all the assertions that everyone knows what a "forge" refers to in the middle of all the threads wondering wtf a "forge" is...
Once upon a time, a lot of software was released/available through "source forge", which is pretty self-explanatory in the context of software publishing. Then a decade ago, SourceForge shit the bed and destroyed its reputation. I'd bet that most of the developers saying they've never heard of "forge" in this context have entered in the industry in that time.
I guess you need to know about the foss ecosystem to know it.