the safeword was always there tho? it's not like she's just now giving him permission to use it. she was saying ''why the fuck didn't you say it if you were hurting'' and hyunwoo goes ''i like pain so whatevs'' bruh. How are you gonna make this her fault lol?
Bruh her mentioning the safeword exists at the end is like asking for permission to use something after you break it.