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.
Kennebec potatoes might make a difference. I have no idea what variety of potatoes McD’s used almost 50 years ago, just that they came in 100-pound sacks.