_As I understand it_ the way a hydraulic disc brake works is by distorting the seal under pressure, allowing the piston to move and push the pad against the disc. Removing the pressure allows the seal to relax to its normal square section, and pulling the pad away from the disk. The seal _must grip the pad_ in order for this to work. Therefore lubricants are undesireable on the piston. As the pad wears, the piston will slip through the seal microscopically to take up wear. If the piston is lubricated, it will slide though the seal and not be pulled back properly from the disc, causing premature wear of the pad.
I suspect, having been a copious lubricator of pistons for some years, that the seals are able to work as designed even when covered in slippery stuff. Certainly, until recently, the rust resistance of lubricated pistons was more desireable than any percieved disparity in pad wear.
I leave you with a snippet (from memory, so the wording may not be precise) from a 1927 Velocette owner's manual.
"If the brakes are too fierce, a light smear of grease may be applied to the linings."