Thoth was defended, just externally.
the station was being guarded by the Osiris, Protogen's last functional Amun-ra-class stealth frigate. Emerging from behind the station, Osiris fired two torpedos at the Guy Molinari. However, it had not seen Rocinante, and Holden's frigate jumped into action. Shooting down both torpedoes, Rocinante moved to engage the stealth ship with PDCS. While the two other ships battled, Molinari launched two breaching pods full of OPA mercenaries, along with Miller, towards the station.
At first, the larger and more advanced Osiris got the upper hand, disabling one of Rocinante's portside thrusters and driving her into cover behind the station. When the station's "dumb gun", a cannon designed to protect the facility from debris and moving asteroids, threatened the breaching pods, Rocinante came out to try and take out the cannon. Osiris put a rail gun round into the frigate at close range, doing little damage but driving her back. The station's cannon destroyed one of the breaching pods and set its sights on the other, but Amos Burton repaired Roci's damaged thruster and the frigate swung up and over the station's habitat ring, destroying the cannon and proceeded to dance circles around the Osiris, riddling it end to end with PDC fire until it lost power and was left adrift. The remaining breaching pod attached and Miller's group assaulted the station.
Source - Expanse Wiki
The Osiris very nearly destroyed the Rocinante (itself an extremely capable ship under the control of an extremely capable pilot), so was probably deemed sufficient defense, along with the static cannon. There just didn't seem to be a reason for more than a token defense on the station itself.