“troop carrier with 200 riflemen”
Those 200 men were artillerymen, not riflemen. Their function would have been to man the big guns at Sumter.
They were not schooled or drilled in the use of shoulder weapons or infantry tactics.
This practice would not begin until mid 1862.
I recall reading that they were riflemen, but whether they were or were not is irrelevant to the larger point of why they were there. They were "reinforcements" sent along on a supposed "resupply" mission that looked suspiciously like a reinforcement mission.
And I would dare say that even artillery men knew how to be a rifleman if they were in the army.