They are required to tell you that they're arresting you (it's unclear if they were actually saying that), they're required to tell you who is arresting you, they're required to have probably cause, they're required to document it.
It's always possible to impersonate police, I'm not sure what your point is?
Wearing what the officers in the video are wearing - uniform with a prominent POLICE label - is sufficient to be identifying yourself as police, and would be impersonating a police officer if a member of the public did it.
Here's a resource on your rights if you're confronted by the police (or any agent of the government, one supposes):
https://www.aclu.org/know-your-rights/stopped-by-police/#ive...