It might be before they were ported to DOS or OS/2, but it definitely wasn't before source control existed (SCCS and RCS were both definitely earlier.)

If architectured properly (big if) you can split up the project appropriately so there is a common core and individual parts for specific OS.

Is it extra effort? Sure. Impossible? Definitely not.

SCCS was created in 1973. We're talking about over a decade later.

Also primitive forking, diffing and merging could be (painfully) done even with crude tools, which did exist.