Earlier quoted context omitted.
Requiring more confirmations decreases the probability that a transaction will be reverted, under the assumption that an attacker has < 49% of the hashpower. If you attempt an attack with 49%, then you have a fair chance of mining, say, 6 blocks before the rest of the network. If you get unlucky then you sacrifice those rewards. But if you mine with 51% then your attack chain is (probabilistically) guaranteed to even…
There's a question that you're missing or ignoring about how long the 51% attack is sustained. Sustaining a 51% attack for one day is very difficult, doing it for one week or one month is proportionally that much more expensive.
Why is mining an attack chain expensive? If you mine on the honest chain, you earn the block reward for each block you mine. If you mine an attack chain with 49% and your attack fails, then you sacrifice those rewards. Suppose you mine say 10+ blocks with 51%+, over the next three hours while the rest of the network only finds 8 blocks. Then your 10 blocks become the longest chain and you earn the block reward for all 10 of them.