by the longest-chain-rule, the chain with the most blocks is considered to be the main chain in Bitcoin. I understand that an adversary needs to have more than 50% of the network's computing power in order to create its chains faster than the rest of the network. Those chains become the main chain under the longest-chain-rule.

However, that implies that the adversary sticks to the network's difficulty setting. If it didn't, it could create longer chains using much less computing power.

I wonder: Why is this setting (an adversary ignoring the difficulty and mining a long chain) not considered? Nodes that freshly join the network don't know about the difficulty and, according to the longest-chain-rule, must choose the adversary's chain.