The 75% he is referring to is if BIP100 was ALREADY activated.
I don't see any activation threshold, best I can tell is that it's effectively 75% since there isn't anything else.
Garzik's BIP100 uses the standard core=95% activation of the BIP.
That's nothing to do with the 'regular' block size changes once BIP100 has activated ... if you understand how BIP100 works.