I'll go with what a few have said and agree that I always assumed that Zaraki's sword was always in a constant release form. Like Ichigo, his power was so immense that he couldn't put a lid on it; I mean, he was so incapable of controlling himself that he had an eye patch made specifically to eat up enough of his overwhelming power to give himself any kind of actual handicap in a fight just so he wouldn't murder a foe in a half second.
The idea that Zaraki of all people could somehow force a seal on his blade is both hilarious and a sign that Kubo doesn't even remember his own story details anymore. Or that he simply doesn't care. But then, I suppose I had proof of that in Diamond Dust Rebellion, where a core part of the conflict was based on something that was impossible given how the nature of their swords work.
I also suppose there was even earlier proof when it came to Tousen, his sword and how in the world he could have two shikai's, but that's a whole other thing.
I suppose though, since I'm forced by either indifference or incompetence (that pace makes me think incompetence) on the part of Kubo to accept this, I'm inclined to agree that it fits him, since it's just a giant smashy thing designed to smash. I mean, at least it isn't a giant Oni style club or anything even more generic, so there's that.
Given the hype, I knew Kubo couldn't deliver... but I didn't expect it to fall that far flat of the impossible goal. There's failing because there's no way you could win and then there's THIS.