Where did you hear that? McDonald's does have specific varieties that are on an approved list, but none are proprietary just to McDonald's. They're mostly russet varieties. And a variety of potato can't be grown by seed and remain the same. They're almost always propagated by cuttings so they're effectively cloned. But it does appear that they do have contracts with farms and then those are shipped to their suppliers (like Lamb Weston) to be processed into frozen fries.
Back in the beginning almost all were the Russet Burbank variety. The other four that were accepted in the USA are the Ranger Russet, Umatilla Russet and the Shepody. McDonald’s officially accepted the Clearwater Russet and Blazer Russet recently to bring up to a total of seven varieties they approve for their frozen French fry potatoes.
Might be different in other parts of the world.
We use varieties such as Pentland Dell, Russet Burbank, Ivory Russet, Innovator and Shepody potatoes which all make our world-famous fries. And as they’re bigger spuds, it means the fries we cut from them are long enough for you to nibble and dip.