I have not seen a think where is says the hacking can take turrets offline. It can just force the ship to allow docking.
In addition, I strongly suspect that ships will be able to counter hack to prevent this docking operation, forcing the Legionnaire to use breaching charges...all while being attacked by turrets that have eaten through the shields on a ship that is probably low in maneuverability, and is probably going to die quickly under sustained heavy fire.
To be frank, the Cutty Steel is probably going to feel like a fortress compared to this ship. Do not kid yourself in thinking this will be able to withstand heavy fire directed it's way. The only way it's going to survive and work is if the objects gets target fixation on something else and ignores you until it's too late, You catch a person operating solo, or after your heavier ships have reduced it's defenses such that it can't actively resist you with ship mounted weapons. Even then, let's say you blow a hole in the hatch. If you are right up against the door when it happens, I bet the laser trip mines that have been planted are going to take out one or two of your lead people in that tunnel, making everyone else a little slower getting out. At that point, just launch a bunch of grenades or a rocket into the boarding tunnel and bye bye boarding party, and maybe even the ship.
I know it's all fun and all to think about actively boarding a ship during combat and taking it over, but I would point back to the old Cutlass Black commercial. You are going to need to disable the ship, get inside, and then hope you didn't bite off more than you can chew.