I can't speak for Celtavian, but my guys will defeat them in detail, recruit allies/summons, poison them, or leverage range/mobility and cover in a fight like this. You don't just walk up to the enemy and start hammering away. Combat as war.
At 3rd level? In a single encounter? From 23 foes? Where a single high damage critical could knock a low hit point PC unconscious?
I get the whole combat as war, but at his level of results in a single encounter, that means that the DM gave the PCs a whole lot of help. Things like range/mobility and cover? Why don't the gnolls have that too? Are they just standing in a line, 200 feet apart, just waiting for a group of PCs to mow them down the line? I keep reading how tough his encounters are, but they must not be that tough if he gets results like this. One cannot get those results without the proper tools and the tools (like environment) are set up by the DM which means that the DM is making the encounters super tough monster-wise, but then giving the PCs an extremely powerful edge (allies, bottlenecks, traps, poison, nearly impregnable defensible positions) to win with. Sorry, even in a combat as war scenario, the DM is setting the bad guys up to fail, not succeed. Why aren't the bad guys using combat as war? Why is that a PC only possibility?
The really nasty DMs are the ones that can send a dozen goblins at a fresh 3rd level party and the PCs just barely manage to avoid a TPK (i.e. the goblins use combat as war). This DM sounds easy because he is obviously seriously helping them in some ways. When something sounds too good to be true, it almost always is.
I find his claims and your claims on this to be totally suspect. If the silver dragon ally helped, it's not exactly a CR 8 encounter, is it? If the players said "is there a way to poison their water supply?" and the DM says yes and 2/3rds of the gnolls are killed or disabled before the encounter even starts, it's not exactly a CR 8 encounter, is it? It's two encounters. One basically to poison them with presumably limited risk (or the PCs would be swamped by the gnolls), and the second with less than half of the original gnoll force. If the DM has 2 gnolls come in every single round, it's not exactly a CR 8 encounter, is it?
Now, multiple encounters with short rests in between? That's a different story.