Earlier quoted context omitted.
> it's just that with 51% of hashpower it becomes relatively cheap to perform such attack The qualitative difference is that with 51% of hashpower you can sustain the attack indefinitely. Other miners might get the occasional block and temporarily take the lead but in the long run you'll always have the longest chain using only blocks you've mined yourself. With less than 50% there's a chance that you can revert a tr…
That assumes that every other miner all mines the old chain. If you and ‘dumb’ miners that just use the longest chain add up to 51% then luck is enough.
No, I was assuming that the other miners continue the longest chain, not the old chain. But they'll include the transaction(s) you were trying to suppress in their blocks; they're not on your side. Each time they mine a block you have to start the attack over again, and the odds are not in your favor. You control under half the hashpower and you need to stay at least one block ahead of the main network. It's not a race you can win in the long term, even assuming a certain amount of luck.