Umm, all taxes are a form of extortion! No matter what country you live in, the state will say "hey, give us money or we'll kidnap you and keep you in a cage", so you pay them to stay out of the cage. That's the definition of a "tax". I certainly didn't agree to this deal between me and the state; the state decided it for me. The state is taking my money without my permission and using threats to do it. Taxes are a form of stealing, for sure, but it's legal because the group that decides what's legal and illegal is the one doing the stealing.

Essentially, it doesn't matter what pieces of paper say about the legitimacy of the IRS. It matters what the people with the weapons and the cages are threatening to do to you if you don't cave into their demands. If the state says it's legal and acts like it's legal, it's legal.