Here's the explanation of why Tau is used: http://www.theoremoftheday.org/Annex/taumanifesto.html

Background of Tau: https://en.m.wikipedia.org/wiki/Turn_(geometry)#Tau_proposal

And a nice animation of Tau: https://commons.m.wikimedia.org/wiki/File:Circle_radians_tau...

I was also confused. Personally, I find the use of tau nonsensical -- especially when it's used alongside pi.

Established conventions should only be changed when there's a clear need to do so. Inventing a redundant constant does little other than add additional cognitive load. It's not like the tau advocates can break backwards compatibility -- there's too much literature that uses pi. The result is that everyone must now remember both pi and tau.

I think tau was used on purpose in that article to avoid confusion with the prime counting function, typically denoted pi (unrelated to the constant pi)

Yes very confused, but I think we're just overthinking it and that tau is just being used with it's normal definition.

tau = 2*pi

I wish the authors would include a small note "tau = 2*pi". Maybe one day it won't be necessary, but for the foreseeable future it would quite increase readability.