So asking for mechanics that would improve gameplay is asking for it to be dedicated? By your logic then having an engine dedicated to graphics woupd make all games look the same.
What? No. The post mentioned the engine itself having the gameplay features.
Look, what currently happens is that engines (generally FPS-based) are sold with a certain feature set (physics, rendering, weapon handling, script type), and then companies buy the engines and
either use them as-is, or modify them to add their own features on top. There's the distinction I'm talking about.
For example Borderlands uses the Unreal Engine 3... but you don't see concepts in borderlands showing up as-is in other UE3 games. Borderlands has things like randomly-built guns with potential elemental effects, respawning bosses that drop unique guns, randomly-spawning special monster varieties, a level-up and skill points system, different special abilities for character classes (phasing into another dimension, setting up a turret, etc.), an RPG-type interface when dealing with NPCs and quests (getting money and EXP as a reward, quest progress markers), vending machines... all this shit is stuff that the team built on top of the Unreal Engine 3.
Now there's also games that use the Unreal Engine 3, and add little to no new features, coming out as generic FPS games. These games all share a small set of similar features because they're features built into the engine, which is why most of these games play the same.
The post mentioned adding a bunch of "actual gameplay" features into the engine, that would just encourage all the games to use those same basic features. I mean look at modern FPS games versus some of the earlier ones. The modern ones (like N64 onwards) DO support a lot more for actual gameplay (concept of a shield, kickback, weapon modifications like supressor, Field Of View changes when using scopes, Depth Of Field effect when using "iron" sights) than existed in older games (like Duke Nukem 3D).
The issue being that since they're built-in to modern engines and thus available to all the games using them, these types of features are used in pretty much all the games,
so they no longer stand out and they seem generic. That's a reason that engines rarely include specific gameplay mechanics; if they did a majority of games would make use of them without many changes.
The engine's job is to take care of all the basic hard shit like the physics and the actual rendering (you know, having an engine that runs in realtime and takes industry-standard model and graphics and shader formats and renders them to a pretty image on the screen is osmething taken for granted for, but it's not easy to write well), all the stuff companies would rather not be dealing with when the time spent reinventing the wheel (writing their own base engine) could otherwise be spent on actual content development (making unique features, building the maps, etc.).
By your logic then having an engine dedicated to graphics woupd make all games look the same.
http://images.wikia.com/borderlands/images/e/ed/Badass.jpg
http://image.jeuxvideo.com/images/pc/a/l/alien-breed-3-descent-pc-003.jpg
Both of those games use the Unreal Engine 3, so lolno that's not how it works.
EDIT: Typos and such, as usual for a post of this size.