They seem to be referring to just the hydraulic motor themselves. You would need at combustion engine to create the 129hp x 4 = 516hp and thats not even conside

They seem to be referring to just the hydraulic motor themselves. You would need at combustion engine to create the 129hp x 4 = 516hp and thats not even considering pumping losses in the hydraulic drive which looks to be significant (20-50%) so now you need a 700+hp engine. Sure gasoline is more energy dense than batteries, but you need an engine (which is maybe 30% efficient), hydraulic pump, hydraulic lines and fluid and then the motors all adding weight and losses. Batteries just need a solid state speed control, wires and electric motors which are all above 90% efficient and lighter especially if high voltage. I just don't see this working, trains had hydraulic drives as an alternative to the diesel electric drivetrains used today and electric won out for many reason including efficiency and ease of precise control. Batteries are improving so quickly in both cost and energy density it will be the obvious choice going forward although they aren't quite there yet for useful aviation work IMO, however hydraulic drive is not an viable alternative based on my understanding.