I think the point was that there have been 2 practice squad elevations for every game this year, so elevating 2 today is the same as the first 5 games. With Gibson on IR we have 52 on the game day roster, so there is 1 less person to make inactive, so Chism is more likely. But we had a 52 man roster previously waiting for Tavai, so I am not sure tomorrow is much different than the previous games.
You are right, if 0 or 1 players were elevated from the practice squad that would make it more likely that Chism would be elevated, but the Pats seem to elevate the max of 2 every game.