No actually it was a space weapon that was controlled from a mobile platform.
I could have sworn that...
Doesn't matter actually. In Van Buren, there was supposed to be a orbital platform that contained ballistic weapons...
Although this add on does contradict canon in several ways, the Enclave could have possibly kept it a secret for some amount of time. After all, orbital platforms do have limited ammunition, and the Enclave troops that are in Fallout 3 could have been a secret division based outside of the oil rig, although this is highly unlikely.