The comment implies, though, that they get a special deal because they are a founder. Neither Wikipedia nor the linked articles say that.

There won't be a source because the details of the current commercial contract between Apple and Arm will be highly confidential.

In any event, it's vanishingly unlikely that Apple gets some special unique rights to IP created in the 2000s as a result of having a big shareholding in Arm in 1991 which they sold completely in the 1990s.

Yes, Apple probably gets a great deal but because they are a huge, enormously high-profile customer who has worked closely with Arm on the development of their latest IP.