I don't think it is as black and white as that, (i.e not everyone will see it so, i for example don't) but yes certainly the logic/principle is valid. Really the worst thing (for the SNP that is) Boris could do is give them the referendum, call their bluff. I doubt he doesn't know that too. refusing it will only play into the narrative, sure, buys him time, keeps focus on brexit, and most importantly makes him look good to his voters. But long term, exactly what the snp want. A chance to secure another majority in Holywood for the next 5 years first, and Then a referendum with more time to 'build' their case.