It’s a newer type of phrase/way to start a sentence. It’s kind of like a way to call attention to or put extra emphasis on the action taking place.

For example, say I got to an ice cream store and try 10 flavors. I could say “not me trying every flavor before getting a single scoop.”

It kinda adds a sarcastic or lightheated feel to the sentence.

Just finished this video yesterday, what an excellent deep dive into something I knew nothing about.

I went to some of the subreddits mentioned to see if any of them posted this video, but it seemed like none did.