The only reason of why they don't have a proper safeword, it's because like tou said they didn't talk about it, had a proper converstaion and figured things out just yet.
This is the uke first time experiencing something like this, he doesn't know why he enjoys so much, feels embarassed by it and conflicted for thinking it's not normal. He's a submissive masochist who doesn't understand/knows what he is. Also, context is important and in this case, it's clear that "stop" was used due to him being overstimulated, feeling extreme pleasure, confusion and being embarassed for feeling this way, it doesn't mean that he actually wanted to stop, he's just blurting it out unconsciously.
I hope this helped and you're right, if they don't figure things out and decide on safeword soon, there's the possibilit that someone will end up getting hurt.
okay the uke falling apart looks sexy ngl, BUT if they're getting into pain play of any kind, even if it's spanking, they need to establish some kind of safe word. Even if it's stop lights like green-yellow-red. I'm really hoping that they ACTUALLY have a conversation about what they want their relationship to look like especially since at first the seme kinda just kept spanking after the uke asked him to stop. otherwise, someone's gonna wind up hurt.