Which would have had been a tradeoff too. More time to market, fewer people on each project, slowed down by cross platform code.
For a company selling licenses for installations, wouldn't having support for all available and upcoming platforms a good thing? Especially when the distribution costs are essentially 0?
It was a thing.
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.
From what I've seen of code developed in the 80s, however, asm code was not written to be divided into general and os specific parts. Writing cross-platform code is a skill that gets learned over time, usually the hard way.
Keep in mind this predates basically ANY kind of source control. It would have been nearly 3x the work.
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.
(Nobody else wanted the job, but I thought it was fun.)
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.