It's because NVC has taken away the natural way for people to express frustration, which is by being short with the other person.

Personally, I don't get it either. No one I'm frustrated with seems to understand that if I tell them in an unhurried, matter-of-fact way. They may say that they prefer me to express frustration that way, but it gets no results. I don't see the appeal.

NVC is most effective if you share your feelings in an authentic and vulnerable way. This isn’t the same as being in the feeling (ex getting angry or frustrated) or staying them in a matter-of-fact way.

The magic of NVC comes from the recipient empathizing with the person making the request.

"This isn’t the same as being in the feeling"

I genuinely have no idea what this means. What are you trying to say here? So I can be in any feeling except frustration when I'm conveying that I'm being frustrated? This sounds like snake oil.