It's actually totally possible with crowdsourcing, and a lot of applications actually work, like Waze.
So people believing it works are not stupid, they can legitimately believe you're pulling that from a common database with an app that lets user signal radar or police cars.
The description clearly says "Your phone is able to detect strong radio signals, just like the ones directed at your car by police when measuring your speed.", and the reviews show people expect it to work by radar detection, not crowdsourcing. Many comments are police officers that tested it with their radar guns (and they of course realize it is fake).
Except, the FTC will bankrupt you with false and misleading claims like these. If it is pitched as a toy, that's one thing but you haven't. You're just lying.
So people believing it works are not stupid, they can legitimately believe you're pulling that from a common database with an app that lets user signal radar or police cars.