Here's the problem: 2 John 9,10 warns against welcoming or allowing into one's home a person who "does not remain in the teaching of the Christ". If a person (a) leaves one Christian denomination (Jehovah's Witnesses) to join another, or (b) leaves Jehovah's Witnesses, chooses not to join another denomination, but still believes in Christ and God, do those words apply? No. They remain in the teaching of the Christ. Yet JW doctrine is to declare a person an apostate for the simple reason that they quit association with a denomination.
The WT article above cunningly links that statement with 1 John 2:19, ("they went out from us, but they were not of our sort"), which applies only to antichrists -- in other words, those who have rejected Christ. Has a person rejected Christ, and therefore warranted shunning, just because they left one denomination? No. Yet the WT article deviously links those two scriptures to declare that a person who "was not of our sort" is an apostate and therefore should be shunned. It's a terrible distortion of scripture.
Any Witness who claims there is a biblical basis for expelling and shunning a Christian who formally renounces their membership of Jehovah's Witnesses is a liar. There is none.