I don't know what could have happened except extreme underexposure. I have shot some pretty old Delta 3200 and got printable results. I figured it would be a waste anyhow, so I just loaded it into my C33 to burn through it. The rolls were about 5 years out of date, and were shot in horrid lighting: a dark apartment with light from a 14W bulb about eight feet away and some light bleeding in from the kitchen. I pushed the film, adding about 66% if I remember (20 min. instead of 12, I believe). I got good prints on grade 4 Emaks paper. Dark, but good pix of a fuzzy fat cat. If I didn't want it to look dark on the print, I would not have shot it in the dark, so.....I was happy.
Based on these few rolls, if I were doing this again, to start my thoughts on exposure, I would probably assume that due to fog, the film is *at least* two full speeds slower than the manufacturer's stated ISO of 1000 (see data sheet). So, ISO 250 to start. The highlights of the film can only be pushed so far; three stops or so in my experience with my main developer (and that is only achieved after toning in selenium, and in situations in which there is enough light to expose so that important areas end up at what would be at least a middle grey tonality on the film with normal development, thus pushable to zone VII with development, and to VIII with toning). Because of the 250 base rating, everything ends up placed higher up the film's characteristic curve, which significantly eats up contrast, and puts an even tougher cap on how far the highlights can be pushed. After a certain point, all you would be doing would be adding midtone density while the highlights were creeping along slowly toward or already having reached max. density. As such, I would say that you should expect to be able to push no more than one stop with old Delta 3200 while maintaining natural looking tonal relationships. So, say 500 is the highest you could rate it. In that case, by rating the film at 6400, you have underexposed it by nearly four stops.
This film isn't magic, as its nickname would indicate. To be able to push it well, it should be fresh, and exposed so that you get at least something important up to what would be middle grey with normal development. You can't push something that isn't there to begin with. You can only significantly push mids and highs.
We'll see what happens with the 45-year-old Tri-X I just shot...I expect a black strip of film with extremely faint, grainy images with almost no contrast...if that.