They did. Their answer to the lawsuit advances that argument. This is a separate motion to stay the lawsuit proceedings and refer it to arbitration per the terms they claim were agreed to.
Or put another way this motion isn’t claiming they aren’t liable because of the agreement to arbitration, they’re arguing that the venue is incorrect based on that agreement. The motion is for a stay of proceedings pending the arbitration.
Edit:
In fact the plaintiffs answer to this motion probably illustrates why this motion was filed in the first place. If you have and arbitration clause but you participate in a lawsuit voluntarily, the courts view this as waiving your right to arbitration. Presumably this is to protect against a case where a side engages in a lawsuit and seeing that it’s going badly for them, tries to have it switched to arbitration later. In order to have arbitration at all, they have to start by arguing for arbitration before committing too far into a court case