My suspicion is that they would copy UA -- Delta One passengers only. They don't give Diamonds access to contract lounges, so why would they for their Delta One lounges.
If they do open a higher tier lounge, the trick is keeping the Sky Club food high quality.
If DL copied UAs exact lounge rules then their LAX “D1” Club wouldn’t be very busy considering UA doesn’t count transcon flights for Polaris lounge access as DL only has 3 other D1 flights out of LAX in normal times (PVG, HND, and SYD).