It could turn out they pootled around the sportive because they were perhaps riding with (unknown to OP) much less fitter friend or relative.
Reminds me of a time doing a 50 mile charity ride with some colleagues. One guy, overweight, rode with knees ou/ seat too low style, but wouldn’t take advice on saddle height because it was comfy. Only rising he did was a 2 mile jaunt into work every day, no other sports or fitness. Hammered out the 50 in not much over 2 and half hours. No problem.
Looks can be deceptive.
I would play it by ear and then see what the rides like. If they are full of bluster and no good, destroy them to make a point, but in a light hearted jovial way. But do come back and tell us if you do end up having to eat humble pie.