It has been discussed and I am on record for thinking it is one of the lamest rules ever. I am honestly not sure what the point of it is.
We can trade for Kawhi because he signed his max contract AFTER his rookie contract was up, but we can't trade for AD (unless Kyrie is involved) because he signed his max contract when he was eligible for an extension. If this is a problem, why doesn't every team just wait until after a rookie's (4 yr) contract is up and re-sign him them (or match him if he is signed in RFA)?
Indirectly related, I also think it is incredibly lame that teams can't sign players (in this case Irving and Kawhi) entering the last year of his contract to the what 'would be' max contract extension next in 2019, and instead can only offer a percentage increase on his current contract. We can already do it with rookie contract extensions, but not players who have a 3+ year contract? It really goes against the idea of the NBA wanting players to remain with organizations long term.