It isn't private, see discussion here: https://www.openstreetmap.org/note/4609657#map=15/50.88224/4.34463

It is indeed behind gates and the gate on Rue Alfred Stevens is currently closed. There is a sign at the door stating that, from the sixth of may on, only the entry on the other end, on Rue du Siphon, will be accessible: https://imgur.com/a/mMuYs2f