Sure, all grades are provisional but everyone will assume that this is 'only' 9b+ unless a bunch more people (or Adam) say it's 9c. There's a phrase "exceptional claims require exceptional evidence" which I think is relevant to grading something 9c. That and the fact that Stefano did it quickly will sway people.
If they used broadly equivalent sequences then it would be two opinions, but he's come up with a sequence that he thinks is noticably easier than the original beta (having tried both), which I think shifts the burden of proof somewhat.
Usually if a route's grade has not been confirmed, this is made clear by stating 'unconfirmed' when the route is reported otherwise there is nothing to stop outrageous claims being made (obvs not in this case). E.g. in the old Lancs guide, a new, unrepeated route had a '†' symbol after it to denote that the grade was unconfirmed.
To take the case of bouldering, there is not confirmed Font 9A...yet. So it would be a prudent if it was mentioned that they are unconfirmed rather than just taking the proposal at face value as it is just that, a proposal. Take 'No Kpote, graded 9A by Charles Albert but downgraded to Font 8C by a recent repeater
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