Insisting that support for unions and support for UBI must be linked is the same energy as insisting that more tightly restricting what food stamp recipients can buy is "helping" them.
But they are completely different approaches to this problem. For example, unions do not protect unemployed people, while UBI protects everyone. On the other hand, a union is something you could create tomorrow at your workplace without waiting for the rest of the country to change their minds.
In some sense, they are competing solutions, because if we had UBI, unions would be less necessary, because everyone who feels abused would have the opportunity to walk away... without ruining their life.
Many people have a very simple-minded view of both politics and the world in general, which leads them to a low-resolution model that lumps things into two massive buckets and assume that everyone else takes their set of beliefs wholesale from one of the buckets.