The GP comment is overstating the genetic differences and pollination difficulties. Most apples are not self-fertile. Pairing pollinators together has more to do with bloom times; some trees bloom earlier/later than others and trees need to have flowers out simultaneously.

Here's some more reading:

https://www.cumminsnursery.com/learn-trees/fruit-tree-bloom-...

https://www.cumminsnursery.com/learn-trees/fruit-tree-pollin...