That doesn't mean the rest of Incantation's isn't also superlative. It just means that OtG is THAT FUCKING GOOD.
If you have one band who releases 5 albums, all of which are 9/10 in quality, and ask everyone their favorite, you'll get a varied response. Take another band who releases five albums, four of which are a 9/10 and the fifth is a 10/10, everyone will pick that fifth as their favorite.
Seriously, go burn through any Incantation album at Random. Diabolical Conquest, Mortal Throne, Decimate Christendom, they're ALL as good as anything Immolation has done. It's just that they came out the gate with a legendary album.
I'd also wager Immolation is a little more accessible for most people. The instruments are more clearly defined, whereas with Incantation it's kind of like listening to it all in a cavern, but that's part of their atmosphere.
EDIT: Another way to put it is like this. If you put all of Immolation AND Incantation's albums together and asked people to pick their favorite, Onward would STILL come out on top in a landslide. Then the rest would be an even mix of people picking Incantation or Immolation albums kinda at random.