Same as a 2 point conversion. Which has a success rate of probably around 50%.
So win probability is going to factor that as - 50% chance you win the game outright, 50% chance you don't. Then of that remaining 50%... what are the chances you stop them? If you don't stop them, whare the chances you stop the 2 point conversion? In either scenario what are the chances you make the FG after to either win or tie the game? If it goes to overtime, 50% chance you win the coin toss, then with the ball do you score or do they?
Like it is just a cascade of what if's - Kicking simplifies it to +8, stop them we win, stop the conversion we win, give up 8, and we get the ball to win.
I think the analytical thought is probably go for it - but game flow wise it felt like... this gives us the best chance to win. Not needing to get the ball and a score to get to OT as well.