It’s definitely the latter. Being a former employee or a longtime developer on Apple platforms gives you a ton of background knowledge. Apple’s frameworks generally have a lot of consistency with each other (probably because of how they do review of new APIs internally), so once you know several frameworks you can easily pick up another. People without that background struggle, even if they’re experienced developers on other platforms.
I experience this myself in the other direction when I try to write stuff for Windows. I’m also ex-Apple so I know the tools and I know where to look to find out what I don’t yet know on the Mac, and worst case scenario I can dig in and reverse engineer Apple stuff that does what I want to do pretty easily. On Windows, I’m just a lot slower and I have to do a lot more hunting around to find out what I need. Generally when I do a new feature that needs platform specific code on both Windows and Mac, I do the Mac side first so I can validate the idea and get something working, and then I do the Windows side second so I’m not having to do exploratory work there, it’s just a matter of trying to find the equivalent functionality and hooking it up. I hear from a lot of Windows folks that they struggle to code on the Mac and they blame Apple for this, but really it’s more just the fact that they have a ton more background knowledge on Windows and they discount the value of this.