A lot of other factors go into choosing the date, I wouldn't read too much into this. If you're of the mindset that a mixed boxing and MMA show wouldn't mesh, then why should Affliction worry about a boxing card taking from their audience?
With or without "competition" from HBO, Fedor vs. Arlovski has the potential to do significantly over 100k PPV buys
P.S. in regards to GB's involvement with Affliction: it appears they will in fact be involved in the back-end production of things, so its safe to assume the deal is about more than just t-shirts.