For what it's worth, notwithstanding that you did put a list of reasons why you don't want to go, the fact that two of them have messaged you does suggest that they want you to go, which shows that at least some people in this group care more about you than you might otherwise think. Also, as much as I entirely understand the anxiety around comparing yourself to others, that is something that you grow out of as you get older (as in, into your 30s), and there is certainly a chance that you regret avoiding a situation where you could strengthen some ties with others later on, when the things that are anxious about now don't matter as much.
At the same time, it certainly isn't antisocial to not want to go. These are people that you have had limited contact with for six years. In my experience reunions of this type rarely do actually result in you rekindling old friendships in a meaningful. Instead they can be nice for nostalgic reasons and can be a nice night or day out in isolation, but they have relatively little meaning beyond that unless you put some significant effort in. It is also entirely up to you where you draw your social circles from. I actually have virtually no friends that I am in regular contact with now (15 years or so on) from my undergraduate and postgraduate degrees. But I do have good groups of friends from school and work. I have relatively few good friends that I've made since having children, but my wife has quite a lot of good friends that she's made since we had children. As a couple, there are other couples that we socialise with who come from a range of places in our lives, both from before we have kids (and now we mostly all have kids) and more recently. This is all individual for each person and each couple. I have been to school reunions, and would go to a university one with certain groups if I was asked, but these events haven't expanded my friendship groups in any meaningful way, despite being decent events where I've had fun. So as much I don't really think there's a downside to going, if you don't want to, there is no harm in not going in my view.