Leon's last 4 training camps have been training for forward pressure wrestlers - Usman x2, Colby, Belal. That's 4 training camps in a row training to defend takedowns, hold the centre of the cage, and keep the fight at kicking range.
He just beat Usman (3rd fight), and Colby by somewhat clear decisions by defending takedowns, getting back up to his feet, holding centre and keeping the fight at kicking range. I assume he will do the same to Belal.
If Leon fought Islam next, that's 5 training camps in a row preparing a very similar game-plan vs a wrestler.
This idea that if Islam gets a takedown he's just going to sub Leon easily or hold him down for entire rounds seems unlikely to me. Volk survived full 5 rounds with Islam, Poirier almost survived 5 rounds - both of them had success in defending takedowns, and getting back to the feet when they were taken down, and avoiding submissions when they were taken down. People talk like the fight is over as soon as Islam gets you down, but it's not the case.
Leon has never lost by submission.
You should generally always favour the fighter from the higher weight class in these match ups unless there's a giant skill gap in favour of the smaller man, or a massive stylistic weakness on the part of the bigger man. Considering Leon has been beating the welterweight grapplers comfortably recently, is on a 12 fight win streak, and the bigger man, I'd surprised that so many people think the Islam is the favourite.
If it does end up primarily a striking match I'd favour Leon to edge out a decision.
Leon gets underrated because he's a bit boring.