I'd agree with this logic if all attempts were created equal. But the looks they'll be getting against a UConn team that has been one of the best in the country at defending the three are likely to be vastly different than the ones they've been getting in the Southern Conference.
At one time, it was undoubtedly true that the best way for a mid-major to spring an upset in March was to get hot from three. But times have changed a lot - there's no stigma now. If anything, the stigma has flipped. Today's top programs would wince at the thought of adding a Terry Larrier or Daniel Hamilton, guys who would have been considered highly coveted talents in previous eras but are now viewed as liabilities.
And so if the stigma has flipped, so, too, does the calculus. At least in my mind, the most dangerous mid-majors in today's game are the ones with p5 talent that maybe didn't align analytically with the vision of a better program. They might not be able to match your A game, but they can absolutely beat you if you go cold. And, at least in recent times, that's what's led to the major upsets - the big boys going cold.
2023 Princeton only shot 4 of 25 from three in its stunning victory over 15-point favorite Arizona. FDU was just 7-23 in its win over 23.5 point favorite Purdue. Even the aforementioned Foreman team only hit 10 of 28 from deep in its win over Virginia (they were only five point underdogs, so not necessarily as big an upset as it seems).
Sure, there are plenty examples of an underdog shooting over their head and winning, but generally those are defense-first teams that shoot the three at a fairly low volume and may only need a bounce or two to turn a 5 of 17 night into an 8 of 17. The best example of this is probably Saint Peter's in 2022, which shot 9-17 against Kentucky (but only 9-34 against Murray State and Purdue).
By all accounts, Furman isn't that caliber of team. It's 200th in adjusted offense and 181st in adjusted defense. Anything can happen in one game, but I think it would take a borderline miracle for UConn to lose.