I'm still not cleat why advertising a session as "mostly (but not exclusively) traditional" (not saying this is not a good thing BTW) raises performance standards?

Not just trad - I think advertising a session as anything is likely to result in higher standards than advertising it as "Gasworks Croft Folk Club, all welcome". I don't think there's anything actually wrong with the Gasworks approach, but I don't think there's anything wrong with encouraging people to raise their game a bit.