You are right. Crisp fried potatoes need to have had sufficient water replaced by oil, and that takes longer than the time to fry them from raw to done. If french fries are made by frying raw potatoes to doneness in one step, the fries will be fine for about one minute, tops, before the remaining water steams out of the interior and turns them soggy.
I worked at McDonald’s in the early 70s, when they still made fries from raw potatoes, and I have made literally tons of fries (on a good Saturday, we’d go through 900-1000 pounds of potatoes). The method then was to peel the spuds in a rotary drum peeler, then slice them, then rinse the sliced fries three times before blanching them in 280-degree oil. After that, the fries had to be allowed to cool before the final frying at 325. I don’t remember if the frying oil still contained beef tallow at that time, but I think it did.
I don’t know how In-and-Out does theirs. I’ve only eaten there a few times, and didn’t pay much attention to their french-fry system.
In n Out slices them in to a bucket of water with a press slice.
I don’t know how long they keep them in the water or if they blanch them like you describe. I don’t think any fast food blanches like that.
Also, as someone pointed out, In n Out use kennebec potatoes.
In n Out’s problem is they get hard fairly quickly as they cool. They’re good when hot, just made.