She didn't give a security guarantee. And even if she wanted she can't.
Japan can't even sell arms to Taiwan right now. Even starting selling arms would be a huge change, let alone a mutual defensive pact.
It's extremely hard to change the constitution of Japan. It's the only constitution that has never been revised since WWII. LDP has been pushing this agenda for decades and nothing really happened.
> It's extremely hard to change the constitution of Japan.
It’s easy to ignore or work around it though, just like it routinely happens for every other constitution in the world.
She just won a super majority in their legislature, she can even amend the constitution now.
Japan's foreign policy about Taiwan includes the notion that an attack on Taiwan is an "existential threat" to Japan, enabling a constitutional reasonning for a war in such a case.