Originally Posted by xliioper

You need to be flying in a Skyteam premium cabin (Premium Economy or Biz) to access based on DL STE+ status as a number here have confirmed. You won't get in simply by booking on KL metal when flying coach if agents continue to follow the instructions they've been given by DL.

According to Sky Club access page, an unbranded Amex Plat card also will not work when flying KL metal (no matter how it is coded). It must be a DL operated flight to enter with Amex Plat (either DL-coded or KL-coded, but it must be DL operated).

However, this conflicts with Amex Plat T&C's on Amex website which says either DL marketed or DL operated is acceptable (according to DL Sky Club access page, this is only the case for the DL Amex Reserve card and not Amex Plat). So you may get in with DL marketed, KL operated flight, but KL marketed and operated would probably definitely be a no.