The lounge was added to Priority Pass recently. Here's the info from their site.
Lounge: AIR FRANCE - KLM LOUNGE
Location: Airside - Concourse A, opposite Gate A22. Passengers departing from other Concourses can access the lounge by train. Please allow sufficient time to reach the departure gate.
Opening Hours: 10:30 - 22:00 daily.
Conditions: Children under 2 years are admitted free - Access may be restricted due to space constraints - Non-smoking.
I've only used it once but found it a bit on the small side at least when I was waiting for my KLM flight and passengers were already there for the later AF flight. It will be interesting to see how many more users this change brings in. I think the BA lounge is actually closer to the DL gates, but it's only open for PP users until 2pm.