It's not really about collective security per se that pisses China off, but the fact that collective security only triggers in limited specific circumstances, (e.g. only when there is any existential threat to Japan as a result of attack on close partner) It's a huge leap (lie) to conclude Taiwan attack is anywhere an existential threat to Japan, so Sanae saying this is provocative to say the least. It's say "theoretically, we can interpret send JSDF into Taiwan contingency as it threatens the survival of Japan" which is remilitarization mindset that does need to be put in its place.
