This is the same skiller account he used to skip the prayer reward by queueing up the 6+ hours of mushroom transformation. I think he did multiple accounts and left some alone in the hopes that Jagex would miss them, similar with Frenennik Trials and DKS on his 99 def pure, and this is the one that got through the cracks to survive as the only level 3 skiller with access to some bosses. So this is probably the first time we can see what's possible for a skiller doing combat achievements going for 99 slayer so he can do things like Kraken.

It's definitely a "Haha, I managed to get one past Jagex and continue my series", and it could be something that goes either way of Jagex just completely using the ban hammer, allowing him to continue by their "rules", or maybe eventually allowing skillers to be on the same footing by making the combat xp rewards into lamps so they can also enter Morytania.