Well duh I'm a Gears fan i should know hence the "implemented" part. Surely they could have added horde even if it wasnt part of the first game.
It's not a matter of if they could implement it or not. It's a remaster of the first game with some additional features thrown in. Extra content, in this case game modes, are bonuses thrown in to the package, but it is still a remaster of the game. They haven't removed a feature, and they aren't ripping anyone off of a feature that wasn't there in the first place.
Besides all that, the majority of Gears 1's maps weren't designed with Horde in mind. Does the first game have enough enemy variety for Horde? Do the COGs have enough variety and tools for Horde? I think people are oversimplifying the amount of extra work required to implement it.