What does it matter what they were in the first place? Are you honestly saying that someone can never change their beliefs? You're right, they were not Christians to begin with. So what? People are allowed to change what they believe.
You are VERY paranoid. You honestly think that a Hindu converting to Christianity because he/she heard about it is somehow forcing or enticing? You must be joking.
I have news for you, once they are baptized, they are Christians, regardless of what they were before.
Would you make the same assertion about Christians who become (as you say) "so-called" Hindu? That because they were not Hindu to begin with they would not be fully Hindu?
That is absolutely insane.