What do you mean supposedly? That's exactly what happened, there is no debate about it.
I never blamed GSP for coming back, winning the MW title, and then leaving again. He put years and years into MMA, being apart of the golden era building the UFC to what it is today, more power to him if he wants to pick his fights and come out for only interesting stuff.
But, the UFC was stupid for giving him that MW title fight. A guy 4 years retired that waited for the easiest MW opponent to rise to the top, there was no doubt in my mind it would be one and done for him. The UFC have had a big problem for a while now of completely throwing away any illusion of structure to title fights and how a fighter gets one, it's all just pure money matches these days. Old names worth more than current wins. Champ vs Champ fights before one or both have even defended a title. That shit bites them in the ass now and they've de-legitimized a bunch of their divisions with it. They have no concept of who the actual talents are that will rise to the top, it's all just who is trending and lets give them a shot and see if they can hail-mary their way to looking like they belong.
So with all that said, if they don't want to make Khabib vs GSP then I say great, it shouldn't be happening. If GSP wants to get back into the pool and earn at least 1 win against a LW top 10, and looks impressive doing it, then by all means make a title fight. But I've no interest in seeing GSP come back and potentially destabilize another division he has no intention of competing in beyond one fight. And I don't blame the UFC one bit for not giving it to him.