Just to circle back and close the loop on this - my example flight (which is today) indeed turned out to be mildly oversold as many predicted. The Y seat map is totally full, while the J seat map still has 4 open seats.

As soon as the gate opens, I'll volunteer and find out whether the open J seats result in op-ups or in rebooking+voucher - either would be fine by me, thank you very much - and report back.

Anyway, to answer my original question - the seat map is a very bad indicator, and it's looking increasingly like the availability may not be accurate either (since there appear to still be a couple of J seats that have stayed unassigned throughout despite J0 - perhaps reserved to accomodate oversale passengers).

In retrospect, it's actually pretty obvious this opacity is beneficial to Delta as well, since it helps to mask the true loads of their flights from the friendly revenue management professionals in Chicago, Dallas, and Tempe