This stat is referring to initial meetings between accepted match requests. Not all (or even most) of those initial connections will turn into an actual co-founder relationship, and founders can of course connect with multiple potential co-founders in parallel.

If matches are 1:1, perhaps they are counting each side as a match (which seems fair.) So they have 2x4500 matches.

Presumably, the matches are compatibilities across the entire sample size. Meaning 1 founder can have multiple potential matches.