You see, this is the gormlessness of referendums. Things change. Democracy isn't a one-time-only affair. A rematch isn't "overturning" anything. Referendums are, in my opinion, profoundly undemocratic precisely because they crystallise things that can't be crystallised.Sure GusB, any excuse to overturn the result.
In Indyref1 we were in the EU and going to maintain the UK; now we're leaving the EU despite most Scottish people voting Remain. I'd say demands for Indyref2 are legitimate, although I think another referendum would be a mistake for exactly the same reason the Indyref1 and Brexit referendums were a mistake.
It's also a manifesto promise so if you don't like it don't vote for the SNP.