He carries a sword inside him, don't forget he can make a sword, whip or bullets out of his own bones.

In the anime he makes his arm bones a sword, his fingers bullets, and his spine a whip, its part of his bloodlimit.

And if size is a problem, remember Asuma uses daggers and he is considered a sword user.
If Danzu was a sword user, so would susuno itachi need to be as well since his susunoo uses a sword in the standard attack as well.