Where the Word of God is proclaimed and rejected by the audience, the believer who so communicated it is to depart, even removing the dust off his feet from such an encounter.
Yes, it is possible to discern antichristian behavior and not associate with it. This doesn’t involve the judgment of that person’s salvation, but may involve wisdom, whcih is preceded by knowledge.
Since Salvation is tied with whether one is Christian or not, the author of this article made that judgement and this is not for him to judge.
You can offer your wisdom on whether Mormons deviate from biblical teachings, that is different from judging whether they are Christians or not. This is where this article shifts in rather extreme territory.