The author doesn’t try to cast doubt on “all of the viewpoints” of the union leaders. He simply tries to cast doubt on the specific argument they made, as described in the first sentence.

The issue isn’t that we can clearly discern the union leaders’ motivation — the issue is that the author thinks they are motivated to make the argument they made. What matters is that they might be motivated to make the argument, not just that we can identify their motivation.

The author treats the fact the leaders have an interest in seeing wages remain high as sufficient to discredit the argument. This is flawed, beacuse even if their argument might be affected by their interests/motivation, that doesn’t discredit the substance of their argument.

The conclusion is just that legislators should reject “this” argument, as in the argument described in the first sentence. The leaders might have other arguments; the conclusion doesn’t concern those other arguments. So the author doesn’t assume those other arguments don’t exist.

The author is responding to a particular argument made by certain union leaders. The author doesn’t assume that this argument is made by the leaders of all unions; just the particular union leaders mentioned in the first sentence.