Some plastic is porous, some isn't. I dunno about XPel, but what he's saying is certainly not impossible. However, in my completely unknowledgeable opinion, this sounds like crap.
Call the manufacturer and ask. If they say his claim is bogus, dont call the detail place back, go there in person. If he doesn't believe you, then have him call the manufacturer while you are there.
If worse comes to worst, you get a letter from the manufacturer stating "if done properly there should be no bubbles" or something like that and show it to him.
But then again... he might be right, and honest. I'm sorry I don't actually know what I'm talking about