See the bottom of that link.
Just because it's frozen doesn't make Yellowfin Tuna sustainable. It's not. Sustainability is based on how much is taken from the ocean vs. how much the fishery can sustain. Even though Yellowfin may be, arguably, the least damaged tuna fishery among red tuna, that doesn't make it sustainable.
The only tuna fishery out here that I know of that's somewhat maintained its production is the Albacore fishery. And luckily for me, I'd MUCH rather have fresh Albacore than Yellowfin any day.