Let me spell it out for you:

BYU doesn't offer a kid a scholarship and hope he doesn't accept it and goes somewhere else (talk about stupidest comment ever)

(Oh, and the fact that BYU is coming in late on these local recruits, offering, and losing them is another bad sign, but I don't want your head to explode so I'll stick to the basic premise of this conversation)

slam dunk
noun
1.
(basketball) a scoring shot in which a player jumps up and forces the ball down through the basket
2.
(informal) a task so easy that success in it is deemed a certainty

Pretty common term even beyond basketball, I looked it up for you so you could see just how stupid the term is, I'm assuming you don't interact much with other humans, because it's kind of a common informal term. Slam dunk is a term normally associated with a high percentage conversion rate (not a 100% guarantee), yes, being LDS, a legacy recruit, or a local player (especially Timpview) means that BYU normally converts a higher percentage of those kids. Not sure why this is such an impossible concept for you to understand, is English your second language?

Just because a kid is LDS, local, or a legacy player doesn't mean they have to go to BYU, but when BYU is landing a lower percentage of recruits they target, it's a bad sign.

Don't worry buddy, I'm not trying to ruin your faith in the Church by saying it's a bad sign when their conversion rate is down on their typical recruits, it's just a bad sign.