The reviewer pretended to be neutral on the forum but was actually shilling for that mobile app.

shilling 是动词 shill 的现在分词形式。shill 原指"托儿"或"帮腔者"，指假装中立实际暗中为某产品宣传的人。作为动词时表示"充当托儿"或"暗中推销"的行为。