I wouldn't be so sure. Grindr tracks users locations pretty precisely, and although many users hide their faces in profile pictures, many more demand a face be sent through direct messages, etc. If you have location and a face, and your target is even remotely popular, some internet sleuthing can put the rest together pretty easily.
How much of this can be validated and proven to not be a masquerade (fake pictures / profiles are ridiculously common, as are bot accounts on the platform) is anyone's guess, but for blackmail purposes, that doesn't really matter.
Given how the app is marketed, and how pervasive it is throughout gay culture in the US (pretty much everyone in that community at least knows about it, whether they use it or not) just being shown to have an account on the service carries a pretty strong implication that the user is seeking a hookup of some kind. Now imagine if the target is married, or they're a closeted politician... it's pretty easy to see how dangerous the dataset can be from there.