We don't know that for a fact.
We do know that Timegate worked on the single player, and a Timegate employee has listed that he did a lot of work on Mission 1. Beyond that it's just rumors and possible leaks. Timegate could have done 10 percent of it, 50 percent of it, or even 99 percent.
We also have an account that when Gearbox got Timegate's work they saw that it was a mess and did a pass over it to get it into releasable shape.
However, it's somewhat unclear just how the single-player is Timegate and how much is Gearbox.
Regardless, it's still Gearbox's fault because they were the ones that contracted the work out, and they should have been monitoring the work.