Read it one more time. He clearly is joking with Pain, teasing him the someone was able to slow down the "invincible" pain. and anyway Obito's word is obviously not worth one tenth of BZ's. He was played by Madara. And BZ played every shinobi including madara.
He wasn't being "sarcastic" - we're talking about the leader of the terrorist group that caused a lot of problems for Shinobi everywhere; do you really think that he'd be "sarcastic" about someone like that?