brompton seatpost is chrome plated cromo steel. you do not want to scratch or otherwise abrade the seatpost surface in any way; that will not help seatpost slippage. a thin layer of fresh grease creates a sealing effect between the tight tolerance of seatpost to frame tube.

also, the application of a thin coating of grease has the added benefit of preventing rust from forming.

if you have rust forming on any part of your bike, frankly, it means you are not taking care of it properly.