Completely false. The probability that a fork will succeed against an attacker with X percent hashpower after k confirmations is (X / (1 - X)) ^ k. There is no term for block time in there. You could argue that with a shorter blocktime an attack is cheaper so an attacker has more chances, eg. with 6s block times an attacker has 100x more chances, but that effect is linear, whereas the effect of adding more confirmations is exponential - assuming 30% hashpower, waiting for 12 confirmations instead of 6 reduces the probability of a fork succeeding by a factor of 161, so you get the same security in 72 seconds that Bitcoin gives you in 3600.