If they weren't visiting my house, I would myob. If it were my home, I would try to redirect/engage the kids, but not care if they kept to their devices. Technically, they're playing with toys, just different types of toys. It's no different if my son went to someone's house and sat on the couch playing with his Transformer the whole time (although, I can't remember that ever happening since my son will always pick partying with others over being alone). As long as they were polite (the post doesn't say they weren't) and respectful (speaking politely, doing other things as asked, etc), I don't see anything wrong with what they were doing.
Maybe they were brand new devices - I'm pretty sure all parents know how kids can be for the first month when they get something new. That might have been poor planning on the parents' fault if that's the case, but still nothing to get in an argument about, imo.
I also view the part about visiting cousins they haven't seen in years a bit differently - now that I'm older, I see it as precious time wasted. But to them, they are visiting strangers with whom they probably have nothing in common. And now they see their uncle as a Judgey McJudgerson know-it-all, instead of someone they might want to know better.
Staying in the house all summer watching tv may be the perfect break for them - they may have had a completely hectic school year, of studying, sports, practices, other things that didn't allow for them to just 'be'. I have a neice who spends her summers that way, decompressing from the school/sports/volunteering/working craziness she has for over 9mos/year.
We had a family visit us for a long day, a couple with a 10yo son. Our son was 3-1/2 at the time and we had nothing for the 10yo to do. He came with his ipod and played it most of the time. Every once in a while, he took a break to talk, or get something to eat, or watch our son do something silly. I had no problem with him playing with the device.
I hope the rest of the visit went much better and maybe everyone can stay in touch for visiting more often.