The Diaz fight was early in Robbie's career (not his MMA prime) and was an anomaly. Lawler had crazy durability and a great chin..in fact, he's still quite durable. He ate a bunch of vicious leg kicks from Melvin Manhoef (a MW former kickboxer), waited patiently then KO'd Manhoef on a counter. He also went a total of 10 rounds with prime Johnny Hendricks without getting KO'd...and Hendricks definitely had more 1-punch KO power than Gaethje. Hendricks probably walked into the cage a good 25 pounds heavier than Justin does.
Gaethje looks really good right now but he's not even a big LW and Lawler has spent his career at MW and WW. If they had a hypothetical prime vs. prime fight, it would have to be at WW where Justin would be very small. That said, Gaethje would beat him now...it would probably look a lot like Gaethje vs. Ferguson where Justin is more active and a step faster than the much older fighter. Come to think of it, Ferguson has actually talked about fighting at 170...he and Lawler would actually be a good scrap for a couple of older, shopworn guys.