I can confirm you cannot shred. Did LFR Ultra again and also Regular Ultra. Not shredable.I'll make it a point to do LFR on my Druid tomorrow and check it out again. All I know is I was pleasantly surprised to be able to Shred Ultraxion the first time I ever fought him due to the fact I'm not behind him, and I did LFR a couple of weeks ago on my Druid and was still able to Shred him. So unless something changed in the past two weeks, you should be able to. The information I've found online, including on Fluid Druid, are saying that it is possible to Shred Ultraxion. Apparently, Druids need to move back and forth a bit to find the spot where it can be done. I've never noticed that issue, though??? For the most part, it looks like Druids just assumed they couldn't so they never tried. But I'll come back tomorrow with screen shots if I'm able to do it, and if I'm not, I'll just post that I couldn't.
To the OP, if it's true you can't Shred on Ultraxion, that guild is not a good match for you. They should know that and not test you on that particular boss. They should test you on a boss like Yor'sahj where they can tell you to just stay on the boss when the adds spawn. My guild has me do that on my Rogue, so it's not like it's going to take away from the guild's effort to kill the ooze (assuming I didn't get the names of those two bosses confused).
The whole Shred on Ultraxion issue could also be getting two bosses mixed up. I know who Ultraxion is, so it might be that I think it's him I did it on but it was really another boss. I'll have to see.