Well - it is even more than that. They basically used Apple style code resources to define PalmOS apps. You used to be able to compile Think Pascal code on a Mac, and some guy has worked out how to use that code resource to convert it in to a PalmOS app with just tweaking the code resources in the compiled MacOS code. It was quite mind bending to me as a 20-something PalmOS fanboy with a day job doing Delphi. This was in like, 1998/1999 or so. I even went as far as emulating MacOS just to play with it. I don;t know if his code still exists online, but the tool was called SARC (Swiss Army Resource Compiler) is anyone cares to search for it.

I don't think Palm did a lot to change the exe format in the early days. And they used the same CodeWarrior 68K compiler that also targeted MacOS at the time.