This isn’t true. It just creates artificial scarcity in the form of the union membership, similar to artificial scarcity of the Bar exam, and college degrees in general, but mere union membership has even less claim to indicate skill, competence, etc.

When unions function as an exclusionary fraternity, they are actually pretty horrible. A good indicator of a well-functioning union is: the union doesn’t have negative effects on people who don’t join, and they are free to do their jobs side by side with union members and nobody cares, nobody judges anyone for their personal choice, no one discriminates on pay or opportunities.