I actually have experience porting games and engines to consoles. If it runs on a development PC (likely Windows), they have the build system and platform layer implemented, which is the hardest part. Porting the content is also an important step, but really only for consoles, which usually have limited memory and power.
Typically the only problem with "PC ports" today is when the game wasn't designed around mouse/keyboard, or when the devs didn't make an effort to optimize it on consumer specs (although nowadays console architecture isn't too different from PCs so there are more optimizations that work across platforms). Another potential problem is when the game gets a lot of last minute hacks to fix bugs in order to ship on a console and those hacks don't survive a platform transition, then the publisher just tells them to ship as is since there's no certification process on PC. Basically, the problems are almost always logistical/business decisions due to a lazy/cheap publisher.
None of that is going to apply to this game. Rockstar has always intended to ship and fully support PC from the beginning. They had the technology, the talent, the incentive, and the time to do it. The most realistic explanation (IMO) for the PC delay is that they're trying to double-dip.
This is far from the first (or last) time he wrote something like this. This was just a regular thing in the kernel world for a long time (until Linus matured a little).
Whether or not it was a good thing is up for debate I think. Yeah, it's very rude and unprofessional (and discourages new contributors who don't want to risk getting chewed out), but considering the importance of the Linux kernel, it's good to know the lead maintainer is doing too much of the right thing than not enough (i.e. being lax with bad code in order to be respectful). I'm fine knowing that a few tech workers got their egos smashed if it gives me confidence that the code powering civilization is high quality.