So I solo'd Legendary Fayum Oasis

To some extent or another, yes. It's more noticeable on Fayum because wood is the first resource they run out of due to both heavy usage of ships as well as lesser amounts of wood, and the AI does not use the market to buy wood. But in terms of time investment it's incredibly time inefficient.

It's why going for early strikes against TCs is so effective and efficient. If you can take out an enemy's TC before they reach age 4, and the objective is to destroy TCs, they won't make more TCs and they'll be stuck in Age 3 for the rest of the game, meaning you don't have to deal with A4 units as well as A4 techs. Repeatedly raiding their villagers is also pretty solid - If you have your own eco up and running and you smash theirs, even if you lose several armies in the process it means they'll have less units when you go for the next round. Also targeting their unit production buildings, ect. As long as you can quickly replace your army with a massive eco, then you win quickly by just suiciding into them over and over again.

AI also doesn't try to replace all destroyed buildings at once, but rather 1 at a time. So if the AI has 10 docks at the start, and you smash all 10 of them, the AI won't try to immediately build 10 docks to replace, but instead 1 dock at a time.

If you have 20 docks of your own and can instantly replace your entire fleet after losing it, then it's easy to win most legendaries.

The tricky thing is to prevent yourself from getting tangled up in their spam - If you just spam at each other, you'll eventually win, but it'll take much longer compared to microing a small strike force to take out whatever needs to be taken out.