So I would... test to see if it screws on. And if it does but you can’t get it all over the barb, take a fine tooth hack saw and cut off the barb including the hex/nut portion flush with the bottom of the skimmer. Then you should be able to use the union.
Well, the outer part does fit. And the inner part would fit, if it wasn’t for that stupid hex nut portion! So now I’m deciding whether I want to take the saw to it or just clamp the stupid thing and hope for the best. My flex PVC hose fits over the barb, but likely not tightly enough to hold water.
I even managed to pry out some of the glue, it that’s a lost cause. You can see glue inside skimmer and outside... it covers the entire threaded portion.