You don't have to "trust" them. You can just read the spec. See what it actually does for yourself.

I have no doubt that Google did not create this out of the good of their heart. They know 3rd party cookies are a dying tech and they need a replacement. But FLoC, their earlier attempt at a replacement, received a lot of very justified pushback for being a privacy nightmare. And they abandoned it, realising their error. In creating Topics, they've done a really good job of coming up with something that can support advertiser-funded business models while still respecting people's privacy by design.