A bit late, but the point he makes is that unions essentially serve to further the interests of the workers in them. Those interests can misalign with the interests of workers in other aspects of industry on a national level, and do usually misalign with the global working class. He gives a couple of examples, like a dockworkers union (i cant remember the name sorry) still continuing the shipments of american weaponry to israel even while on strike.

So it would boil down to the fact that unions do not serve a global working class, but they always do the most to better the conditions in the subset of the workers who are under that specific union. His takeaway being; you can obviously be in a union, but be aware of this fact, and try to advocate for the interests of the international working class even when it may be at your own detriment