Of course, the police would be able to infiltrate this with a bit of effort.
They can't. The app uses some kind of location data to silo information. So for example if they want to make an action in Tarragona, people from Barcelona is not going to know about it.
It is likely, seeing some threads of people who have analyzed the application, that the administrators, or whoever is behind it, will be able to know how many people are available in a particular location, and isolate that information in a kind of opsec.
It's less clear cut when it becomes more of a "barrier has been breached on this street" along with "cops are moving to this other street" in realtime.
> the movement Tsunami Democratic has been confirmed as a criminal organization driving people to commit terrorist attacks
So any material support to a criminal organization is criminal per se in most jurisdictions.
Sounds like a bit of welcomed transparency to me. The Spanish police, concerned citizenry, etc. can use the app just as much as anyone else, to prevent things from getting out of hand.
(*Edited for consistency with newly-provided info in this HN thread.)