KE does allow overweight checked bags but the max weight of any piece of checked bag is 70lbs, to my knowledge. This is for their own tickets. If it's a Delta ticket, my understanding is that Delta's rules and standards would apply and KE would just help handle it on Delta's behalf. And to my knowledge, Delta also allows overweight checked bags.
Family flew a bunch of KE code shared/Delta operated flights last October. The routing was RDU-ATL-ICN-China Mainland. Ticket number was KE 180 (NOT DL 006). Check-in was with Delta. A fee was paid for an extra piece of checked bag above allowance per KE's rules and standards. The fee was posted to my Amex Plat card as "DELTA AIR LINES", and I was able to use my $200 credit with Delta. So it looks like whoever checks you in matters since the fee is collected by them. If OP's check-in is with KE, then your fee would be collected by KE and you may see "KOREAN AIR" on your Amex Plat statement, so the $200 credit with Delta wouldn't apply here. But if the check-in is with Delta, then I believe you should be able to use the $200 credit.