Earlier quoted context omitted.
How do you merge changes between the source trees? Keep in mind this predates basically ANY kind of source control. It would have been nearly 3x the work.
3x the work may still fall under reasonable cost. 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.)