Well, if you believe that, then his statement makes more sense. I don't believe for a minute that every islamic terrorist group would be deterred from nuking us if they thought we'd nuke Mecca. Some wacky imam would claim it was the ultimate matyrdom necessary to start the final jihad or something.
I also don't believe that all of the terrorists are devout moslems. I'm sure many are, but I suspect moslem extremism is just a convenient cover for some of them who just hate the west for more secular reasons. Saddam and Assad are perfect examples of that. For someone like that, a nuke on Mecca would be like hitting the jackpot. They could care less about the religious implications, but would love the outpouring of anti-Americanism.
Much better to scare the crud out of the governments by threatening to nuke any that tolerate the presence of terrorist groups on their soil. Secular or not, no government wants to get its capital city nuked. Especially if they're in it.
Travis sums it up way better than I can.
Mecca IS Islam, in a way it is difficult for Westerners to comprehend. Two of the "five pillars of Islam" are based on worshipping that black moon-god rock called Al-Hajarul Aswad in Arabic. (5x daily prayer to the moon-rock, and the Haj or pilgrimage to visit and worship the moon-rock in person.) Muslims see the black stone not as symbolic, but as a physical manifestation of Allah on earth, a chunk of Allah fallen to earth. The black stone has the power to suck all of the sins and evil out of men performing the Hajj pilgrimage. (Interesting to note that Islam’s holy of holies is packed with all of the sins of all of the Muslims who ever made the Hajj.)
Devout Muslims believe that the black stone represents all of Allah's power and glory. They believe, sincerely, that if the USA launched rockets at Mecca, Allah would swat them away like flies, or turn them around to hit New York or Washington.
If Allah could not stop the”infidel” bombs, and Mecca was turned into a crater, this would constitute absolute proof that Muhammed was a fraud and a liar, and that the moon-god rock had no power after all.
This would mean the end of the entire cult of Islam. Their cult could not adapt to or respond to or overcome complete, visible proof that Allah was powerless to stop the infidel weapons.