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For context, it looks like this is the original tweet from one of the video creators: https://twitter.com/shivakilaru/status/1372941623609282561

Cease and Desist over Instagram.

Is that even recoginizable in court as legit?

For most actual legal doctrines possibly classified as standing in for "legit", yes.

Actual notice ("I sent it, you acknowledge you got it") suffices for most notice requirement, regardless of circumstances of sending and receiving.

Some statues/rules are more specific, but it's impossible to answer that without a more specific question.