Well, there was one Confederate assault OTL against Fort Stevens in 1864, intended as part of an effort to try to force Grant to divert forces from the Overland Campaign to defend Washington and Maryland. AH writers could be simply playing off of that: just picking a familiar name, or assuming that whatever the OTL reasons for raiding Fort Stevens were, they probably also made it a good spot for serious assault.
Also, take a look at the geography of the defenses:
Fort Stevens is conveniently along a major road leading directly into Washington from the Northwest (so Confederate attackers could cross the Potomac upstream where it's narrower and lightly-defended rather than having to try to force a crossing directly into DC's main defenses across a bridge or cross further Southeast where the river is patrolled by warships.
Fort Stevens is also relatively unsupported by other forts, compared with the rest of the defenses. Based on the map, it looks like in most places, the forts were deployed in depth so attackers would need to fight their way through several rings of defenders to reach Washington, whereas Fort Stevens is the only fort defending the Seventh Street Road approach. It's also a mile away from each of the neighboring forts, whereas in most other places, the forts are spaces half a mile apart or closer.