Federal officers can make warrantless arrests if they have probable cause to believe the individual has committed a crime, just like normal officers. It's hard to tell if these arrests meet that threshold, but it's at least possible that all laws are technically being following here.

Again, that's not to say this isn't a bad situation. I'm explicitly advocating for changing the laws that allow this to be even possibly legal in the first place.

Sure, if they had probable cause. Do you think that those federal agents just witnessed that a federal crime was committed?

In the video I’ve seen, there’s not enough context to tell whether they have probable cause. Which by the way does not require that they “just witnessed that a federal crime was committed”.