It only means they are not releasing these seats for FCM upgrade offers currently. Perhaps they will at some point, but you never know. You can see the same thing on domestic routes. In one case, I have a PHX-JFK flight a couple weeks out with only 3 seats occupied in F and no FCM offer (other seats are blocked). The fact that the flight is also J9 C0 D0 I0 Z0 is likely not a coincidence.

Depending on equipment, they might be blocking of PS FCM upgrade offers out of concern of a possible equipment swap (to a non-PS aircraft). The 764's should be safe, but the PS cabin is not very big and DL could just be waiting to see how demand firms up before releasing PS seats for FCM offers.