Forking and merging is a social phenomenom. Sure git makes it easier, but nothing stopping anyone from just copying and pasting as appropriate. Not to mention diff(1) was invented in 1974, and diff3(1) in 1979, so there were already tools to help with this, even if not as well developed as modern tools.

I'm also pretty sure cross-platform code was a thing in 1983. Maybe not to the same extent and ease as now, but still a thing.

Fork just means two groups start with the same code and work independently.

It was a thing.

You make a copy of the files and work on them and that is a fork.

You'd need a Beowulf Cluster of Apple //e's to run an Electron app in 1983!