To break up a riot, I think you are right and the insurrection act must be invoked, but selectively picking up people based on suspicion of violating federal laws can be done without it, I suspect. In these days of ever-present cameras, social media and electronic communications, I doubt it would be difficult to establish probable cause for specific individuals violating or conspiring to violate or abetting violating federal law.
I agree with you. I live in Northern Virginia and there are all kinds of federal police here, such as Park Police - there are something like over a dozen different federal police agencies here and they all have the authority to pull me over for any reason - even on a local street. I don’t think they ever do, but they could if they wanted to, at least according to the Washington Post.