The seal divides the piston into two areas one side of the seal has brake fluid and the other side is exposed to air a water from without. Directly under the seal is ideally nothing, but to help it slide during assembly, we use some assembly lube.
The piston pitting is predominantly on the piston side exposed to water, salts, and atmospheric impurities, notably oxygen.
Once the chrome plating is breached on the piston, the far more reactive underlying steel is exposed. To arrest further corrosion the silicone grease fills to pits, blocking water and oxygen access to the piston's base steel, arresting deterioration.
Clearly if the pitting is excessive, silicone grease cannot take the place of steel. But, as the silicone does not combine or dilute with anything, it is not washed away by water or brake fluid.
Ideally, you do not want any silicone in a DOT3 containment vessel. But, if the piston pits are small and under (or beyond) the seal instead of inside the DOT3 vessel, silicone grease does little harm. However, if you must have uncompromising ideal conditions, then get yourself a new piston.
Also, be aware that it's not just the piston beyond the seal that is environment exposed, the aluminum caliper walls are as well. A thin coat of silicone grease there, arrests corrosion of the caliper body. Remember, the grease stays put unless mechanically pushed of placement site and isn't diluted with anything. A very very thin coat is an excellent water shed. And, if it sheds water, it blocks oxygen (therefore corrosion), too.
Cheers,
