I am assuming that you know the character Grima Wormtongue in Lord of the Rings - and the negative and poisonous influence he had over Theoden - the King of the Rohans.
Johnson describing the Benn Act as the Surrender Act simply feeds the anger and division. He has said now many times that he will continue to call it that because he feels that that is what it does. Likewise, given everything we know about Cummings - and indeed from his own mouth on Brexit and the Leave Campaign - I do not see it as inappropriate to describe Cummings as Wormtongue - as to me that is precisely the character he is.
In case you hadn't noticed.... "Hobbit"