I completly agree i was thinking the exact same thing when he said those words, like "lolwut? greatest fight ever, my ass". It was a great fight, no doubt but the greatest ever, noway.
I think Rogan just got caught up in the excitment and called it the greatest fight ever then probably realized he was exaggerating but it was too late, would have been stupid to come back on it mid interview and say "maybe not the greatest fight ever but a great one"
It was a great fight, nowhere close to the greatest ever though.