Looking at the regular season and making projections for a LeBron team isn't very accurate. It isn't a secret he coasts through large chunks of the regular season, but come playoff time he always locks it in, just as he did this year. And it isn't just LeBron that is a fairly recent trend, which is why sites like 538 can do a decent job predicting the regular season but a terrible job projecting the post season. The stats just don't correlate when the players are truly locked in. The Clippers tried this this year and failed. The Bucks never really had Giannis play heavy minutes and he couldn't ramp up either so it doesn't always work, but for LeBron it does just as it did for the Warriors with Durant. The C's that year were on a mission all season long and played like it all season long so of course their drtg and ortg were great.
KG was on the downside. His last 1st Team All NBA before that season was 2004. He was on the 2nd Team in 05, didnt make a team in 06, and was 3rd team in 07 and he was fully healthy those years. 08 was his last all NBA team (though obviously was injured in 09). He got a huge boost for anchoring the C's defense on the best team in the league, but statistically he wasn't there. It was clear he had been losing steps for years which is why the Wolves were so bad his last couple seasons there. The 2nd best player in the league doesn't go 31-45 and 32-44 in consecutive seasons. 08 was a magical season, but as champions go that team isn't among the best. The 20 Lakers aren't either, but I do believe they would win a series between those teams.
And I do think it matters a bit on the rules on some level. If the series was today, Perk would barely have played (like Dwight and JaVale). If it was back then, Dwight and Javale would have played a much greater role. I think KG would have a more difficult time guarding AD with the faster pace 3 point oriented game of today, while conversely AD would struggle a lot more in the slower inside out offense of the last decade. Pierce, Allen, and James would have been fine in any period.