He used a sockpuppet account to post on the forum.
Tips
usage
马甲 has two distinct meanings: (1) a vest/waistcoat (regional, mainly northern China), and (2) a fake online identity / sockpuppet account (internet slang, from the phrase 穿马甲 'wearing a vest' = hiding identity).
culture
The internet meaning comes from a famous sketch by Zhao Benshan: "别以为穿了马甲我就不认识你了" (Don't think I won't recognize you just because you put on a vest).
