It's here:
https://github.com/argriffing/statsmodels/blob/9ed378d6836e347c89b2f2c2487f445b09db29ee/statsmodels/base/model.py#L507
The bounds are always set to `None`s because we only want L-BFGS, dropping the -B (boundedness) from L-BFGS-B.