"And as far as I understand, this means that so long as you stay short of decompilation - you can reimplement as much as you want."
Yes, but my point is that.... go on github, you'll find tons of decomps. And many more done just privately too. One of the No Man's Sky devtalks start with "yeah we decompiled the terrain generation from this other game, implemented it in our prototype, it didn't work okay, here's how we've learnt from it to make something better". This was in 2016. More recently, this has been going on way more openly, even full AI-assisted decomps thrown up onto GitHub casually. It might be the letter of law or included in Terms of Service but no one cares really.