No parent comment criticizing unions does it without the implication that they shouldn't exist. This is most easily seen by the total absence of any mention of a specific union, thus direction the objections towards the concept itself rather than any specific version of it.

All unions, as a general concept, tend toward corruption. Just like corporations do. The concept itself is flawed. But flawed does not mean it shouldn't exist.

Unions are a way of concentrating the power of labor.

All power corrupts. The observation proves nothing about unions specifically. If you take away power from unions it just goes and does its corrupting somewhere else.