you simply can't pick one verse out of the bible and justify or base an entire religion on it, and disregard the rest.
It's true that everyone who believes that Jesus is the messiah is speaking the truth.
But unlike Islam, Bahai and Mormons, etc... they all do not know Jesus and/or his gospel so they invent another Jesus by the same name (due to supposed added revelations of who Jesus 'really' is or what he 'stands for') and say that 'this jesus' is the 'real' messiah.
So there is your answer: They believe A jesus is the messiah, but not Jesus as spoken of and revealed in the gospels.
If they disagree with John's gospel or any other of the four, they are not agreeing with John in the context of his letter. So John wouldn't agree with those religions either.
As Jesus said "if you love me, then you will keep my words"
those that alter it, do not have respect for truth and no love for God.