Because he's already the highest paid Coordinator in college football & he just signed an extension last year, he can't leverage LSU for more money by using a small nothing Moutain West program like UNLV to get more money.
Aranda is not a LSU lifer & he's not the reason they went undefeated & made the playoffs this season, Joe Brady is, therefore if his agent is throwing it out there that he might leave to UNLV, LSU doesn't have to panic & pay him more when they already did last year. If he were using another SEC program or another P5 program as leverage it would make more sense, but UNLV? Nah, LSU would just go hire another DC to take his place or simply elevate Corey Raymond from within as the DC in waiting.
If Aranda leaves that's easier for LSU to not have to pay a DC $2.5 - 3 million a year (which is basically Head Coach money), I'm certain they'd prefer to pay that much to keep Brady or would be perfectly fine with paying another DC or Raymond $1mill to do the same job.
There's no leverage for Aranda by going to UNLV, that program hasn't won 10 games since literally the year 2,000 & has been a perennial loser for the majority of their history. Which means he would likely fail there too, so if he's trying to use them to get more money all LSU has to do is say go ahead & fail hard at UNLV you'll only lower your own stock as a HC.