OP - I can see where you are coming from, but I don't think you can tell guests what their children can and cannot do.
However well-meaning, and even if you are right, giving other people parenting 'advice' always gets taken badly, particularly if it is done via email or text. And however nice your text was, no matter how many smileys, the meaning of it was obvious.
The one and only time I have had to unfriend someone was when she emailed me to give me advice (that I hadn't asked for) on how to deal with my 3 year old DD having a tantrum. Yes she was well-meaning, she may even have been right (we all have good and bad parenting days) but her email made me feel like crap and I decided that being a parent is difficult and guilt-ridden enough without feeling like your friends are judging you as well.
Your friend may even be aware that her children have too much screen time, it might even be an issue she is struggling with, but no-one would take kindly to having someone else judging them on it.
I have noticed from your posts that you haven't liked it at all when other posters have made assumptions about your family, or made suggestions about your parenting. Well, that is just how your friend is likely to feel now!
I also thought it interesting that whilst you are obviously not into much screen time for kids, you thought it was OK to communicate something fairly sensitive to your friend via a screen (text)? In my experience text and email can easily get taken the wrong way when used for sensitive issues. In person or phone call is much better.
For example, we have long-standing friends who come over for lunch and their children are very picky eaters, but to me the friendship is far more important that whether or not their kids like my cooking. To avoid any awkward situations we usually have a chat on the phone beforehand, I'll mention what we are having for lunch, and what their kids might be likely to eat. We have a bit of a joke about it! Their kids will grow up, they won't be picky eaters for ever, and I'd like to still have my friends at the end of it.
FWIW, I do think their son was very rude to walk in the house with his face in a screen and I would never ever let my children do that, nor would anyone I know. I wonder if it was a reaction to your text?
Re screen time. As you asked, the amount of computer time your kids have doesn't seem a lot to me, but it is your family and your decision, as long as it works for you that's fine. My kids have more than that, but in winter it does tend to be more than in summer.
Re your 4 year old's tantrum. I think the computer screen is a red-herring here, it wouldn't be unusual at a get-together with a mix of ages for a 4 year old to get a bit bothersome whatever games the older ones were playing, and have a tantrum, whether it is a computer game, lego, board game, den-making, crafts, whatever. The number of times I've had to intervene over the years because the youngest one has 'wrecked' whatever game it is the older ones were playing!
At the end of the day, you have make the decision - whether or not the friendship is more important to you; or the children's interactions / your kids access to computer games is the most important.
If the former, you just have to let it go, and hold your tongue, try not to be bothered by it. If the latter, you have to either lose them as friends, or arrange to meet adults-only, or at a pre-arranged activity (e.g. soft play).
However the damage may already have been done, sorry....