The Trump administration has been routinely embedding other agencies like the FBI in ICE operations:
https://www.nbcnews.com/politics/justice-department/ice-oper...
Impeding federal law enforcement officers is a crime - to which other federal officers (not sure about ICE) have the ability to make an arrest.
Yes, you're required to comply with law enforcement. But if you're required to comply with unidentified law enforcement, we're gonna have problems.
Just as in many jurisdictions you can be arrested for Resisting Arrest as the sole charge. "We weren't going to or had no grounds to arrest you, but since you resisted[1], we're now arresting you for that."
[1] for varying definitions of "resist"
In the video I saw, Lander wasn’t asking the agents to prove that they were law enforcement. He was asking if they had a judicial warrant to detain the person he was holding onto. He did seem to believe they were law enforcement.
The problem is, ICE doesn’t need a judicial warrant to detain someone suspected of immigration offenses in a public space. Lander didn’t seem to know this, or at least feigned ignorance.
What do you do with someone who doesn’t know the law, and is actively interfering with a planned arrest? Briefly detaining them seems like a reasonable path to get the job done.