The only thing the officers told the guy is "We're investigating the protest that happened". Then they asked him questions about the protest organizers and if/how he knew them. It's unreasonable to then say "they're investigating a crime"; they're obviously investigating protected speech and public assembly. They even claim they are investigating such. At that point it's actually reasonable to expect a source for the (counter-)claim that they're really investigating a crime.
> I’m not making a specific allegation
Well, except "they're investigating a crime". They are not investigating a crime. They are investigating. Whether or not they are investigating a crime comes down to whether or not a crime was committed. To say they are investigating a crime means you know what they are investigating.