In the Markan passage, the demon in the man is not "confessing" that Jesus Christ has come in the flesh so much as the demon is merely stating this fact. Is stating something the same as confessing? The objection is forcing these two words to be equivalent when there is no reason to.

Also, the context of 1 John 4 is different than that of Mark. John addresses his readers in verse 2: "This is how you [ie the receivers of this epistle] can recognize the Spirit of God....". The author of 1 John's exhortation to the readers of the epistle is giving members of the Church a way for them, mere humans that they are, to distinguish between the Spirit of God and lying spirits.

The Markan passage does not deal with anything of the sort. It merely reports an encounter Jesus had with a demon. As such, it is a category fallacy to compare these two passages.