the referee didn't stop Overeem from incurring more harm. in fact, the referee was too fucking slow to get there to save Overeem had Rozenstriuk chosen to punch him again.
the fact Rozenstriuk
turned around and began to walk away and celebrate before the referee even did anything should alone, tell you that it was a bad stoppage. not to mention that Overeem immediately gets back up.
the ref should've been waving the fight off as he rushed in, OR waved the fight off as soon as he got there to break it. but he didn't, he waited until Overeem stood back up and wobbled over to the corner before he saw his lip and actually stopped the fight.
basically the fight was stopped because the referee was disgusted by Overeem's cut, not because Overeem wasn't intelligently defending himself or because Rozenstriuk was unloading power strikes on him.
nobody is arguing that the cut isn't bad or that it would've been horrible had he continued to take damage or that it would've been fair to stop the fight if it occurred with more time on the clock, but 5 secs, the dude is still awake and conscious, and the other guy has already walked off is a bad stoppage.