Exploits are cheats but they are varying degrees of unfairness.
Fast shoveling is abusing an exploit to dig faster than intended. It's cheating but the impact is negligable and not likely to be on the developers radar. Getting to map locations in insert_shooter_here that players shouldn't be able to access is exploiting issues with the map geometry and is usually defined as cheating, even if everyone can easily do it.
People are more likely to consider glitching into a map as a more egregious offense than fast digging because of the direct impact it has on player interaction.
That is what we are seeing with double gun. It's an exploit, and therefore cheating, and because it directly impacts combat as opposed to peripherally it gets more attention. Don't fool yourself into thinking both fast digging and instant double gun are intended, perfectly fine to exploit, and not subject to being removed though.