That's a cue to get people to stop trying to get the bar off the ground in the DL with immediate emphasis on hip extension, rather than mostly knee extension with a bit of hip extension - which moves the bar in approximately a straight line up. The reference/similarity to leg press is more to give people who have experience leg pressing a conceptual idea of what this means, since it's tricky for many to grasp.
I don't think it follows directly that leg press is good assistance for the DL or lower body in general, but I also think it potentially can be. You could just squat or DL lighter, but there may be something to be said, under some circumstances, for getting some work to the legs in a way that uses multiple joints over a relatively long and relatively repeatable ROM, without taxing the back at all. While my inclination is usually to just squat or DL at the appropriately lighter weight needed to have the intended effect, I wouldn't discount it absolutely altogether, either.