You might be correct that varience has a near-zero effect for lower-confirmation transactions, but still remember that shorter block times give less varience. So someone with 49% has a near-zero, close enough to zero, chance. The actual probability of getting 7 blocks in a row is less than 0.6% for a 49% holder. The actual probability is smaller, because you aren't rolling an infinite amount of dies.
Someone computed what the actual odds were. Even for a 60% holder it was a small fraction of a percent. But it wasn't zero for below-51%.
So it isn't that you
can't create a longer chain, it's that it's extremely unlikely any attempt to do so would succeed, and even if you have 75% of the power the variance is much higher than I think you're assuming: look at the luck charts of large pools if you want proof of that.
A 60% holder can, however, use a stronger attack - simply fork the entire blockchain. A 49% holder, trying to do that, will not be able to sustain it for long. Varience is higher when difficulty is higher, however, so this is hardly an argument FOR bitcoin.