Five-foot reach. The slaad must be adjacent to strike.The Slaadi are "large", right? That should give them enough reach to be able to attack from a mostly-hidden position, and any PC wishing to attack in return has to step right in.
And sure, the PCs might see bits of the Slaadi poking around the corner, but it would still count as good to great cover vs missile fire and the only PCs that can melee-attack are those one or two who can fit in, and thus be attacked in return.
Cover is a +2 or +5 bonus, and with the right feat, no cover at all (I think the Sharpshooter feat is very, very poorly created).
For melee purposes, it's a +2 bonus to AC.
The rogue is rolling with advantage and +9 to hit against a AC 21 with superior cover. That's not enough to protect the slaad very well.
Just to illustrate, the rogue can see about half the Slaad from his sniping position here. It might be a little less, but that's still a LOT of the slaad he can see. That's half cover, or a +2 AC bonus. (The slaad needs three quarters blocked to get the +5 bonus to AC).