First of all. That massive slaughtering was a huge mistake, even experienced whalers (including the foreman where it happened who wasn't a part of it) in the Faroe Islands say so. Way too many dolphins to way too few hunters. That one was certainly barbaric as they were struggling for way too long.
Second. Nothing's showing a general gain in this. Certainly this one catch will make 2021 numbers much higher than normal (more white-sided dolphins than the last 12 years together), but there is regulation to keep it at a level where it doesn't in any way endanger the animals.
Third. If grindadrap is to be disallowed we would need to see a ton of slaughterhouses and farms around the world shut down as well killing a lot of meat industries. If that's wanted, that's certainly okay and we should generally treat our animals better, but as long as billions around the world eat meat (myself included, though I try cutting it down a bit) we have animals treated poorly all around the world!
Don't get me wrong, I'm not a huge fan of how it's done, but I just don't see it (once again with that single situation as an exception) as worse than many others treating chicken, cows, pigs and other animals bred for eating.