Still say it's more spin off than sequel if only because of the shift in focus and tone.
What shift in tone? That the world is awful and it sucks to be Kaneki? In case you missed it, he literally was a corpse with both eyes gouged out at the end of the first one going on about how his story was a tragedy, only to have this one start up with him mind wiped and made to follow the hunter side, where things still get worse and it still sucks to be Kaneki. There's no shift in tone whatsoever. There's no shift in narrative whatsoever, the big plot hooks still mirror and reflect the loose ends from the last series. There's a minor shift in focus to the hunter side of things, sure I grant that, but that's consistent with a similar shift in focus from the coffee group to Kaneki's Ghoul Death squad. And at the end of the day, there's no 'real' shift in leads either, baring again the memory wipe thing which again they might have just reverted, maybe.
All of that is consistent. This is a continuation, plain and simple. You have one minor point here you can indeed argue for spin off, but at the end of the day literally everything else is pointing against you. You might have more leeway if we were discussing Immortal Regis's betrayal of a continuation, that changed everything from plot, tone, characters, to even the friggin lead and only later managed to actually properly tie the two together though even then it was too late. Here however you have no leg to stand on.
Edit: Actually neglected to debunk your Batman comparison, because really it's kinda self explanatory by itself given the vast amounts of differences involved in these two cases, much less two mediums, but just to be thorough, Tokyo Ghoul literally went from Concluding and ending one series, to starting up the same story again. Your Batman example not only has multiple writers and books involved, all running at the same time, but also does occasional one offs and what not. Yes you can definitely call those spinoffs, they run concurrent to the story, similar to my Fairy Tail examples earlier. Here however we literally have one thing ending, and another picking up where it left off. This is a continuation. No ifs ands or buts.