They were chosen "in Him"...to be placed in the body of Christ, before the foundation of the world.
The verse you may be looking for that addresses the bolded part of the question directly, is 2 Thessalonians 2:13.
No, he's not changing the phrase, because that would be twisting the Scriptures.
To me, he's adding the details of other passages together, and making significant connections about that which is all understood, together, not separately.
What I see him doing is expressing all that encompasses election and what it means to be elect.
That includes everything that is said about a believer..."found in Christ" relates to being "chosen in Him" relates to being "foreknown predestinated, called, justified and glorified" ( Romans 8:29-30 ).
For example:
Believers ( John 3:16, John 3:36, John 5:24 ) = Christ's sheep ( John 10:27 ) = elect ( Romans 8:33, Colossians 3:12, 1 Peter 1:2 ) = beloved ( 2 Peter 3:8 ) = "us" ( numerous passages in the New Testament epistles ) = "the righteous" ( the Psalms ) = "the wise man" ( the Proverbs ) = the prodigal son = "wheat" ( Matthew 13 ) = blessed = "of God" ( John 8:47 ).
Similarly, to be "found in Christ" = "elect" = "chosen in Him" = "found favor with God" = "faithful" = "conquerors" = "clothed in white" = "washed in the blood", etc.
Leave it to a Calvinist to get ALL of this from on phrase, "chosen in him before the foundation of the world," from one verse in the Bible.
Chosen in him = Found in him; nowhere is this in scripture