In my somewhat limited but relevant experience, the amount of platform specific bugs is indeed that low. I mean, there’s of course a layer of platform-specific low level stuff which is highly subject to platform specific issues, but once you go above that layer and into game code proper, most bugs are just bugs.
I didn’t fix 400 “Linux-only” bugs, but I did fix dozens of “seems Linux specific” and “only happened when at least one Linux client was connected” bugs, and a grand total of 2 were caused by platform differences. And of those two, zero were Linux specific. The platform difference in this case was about how different compilers optimise non-crashy types of UB.
Of course, we don’t want UB at all so the fix is to remove it.