Exactly as stated above: ball boys and ball girls are no longer permitted to give the ball back to players.
That's the rule change we're discussing here!
After a quick search, I've just found this which I'd missed before:
https://www.brightonandhovealbion.com/news/3943923/premier-league-change-to-multiball-system/
I was oblivious to the changes on this.
I thought Brentford just had a particular setup on Wednesday night until I saw the same operation in place at the Amex on Saturday.
It seems players have to either use the ball that went out of play or pick one up from a cone. Ball boys and girls do not...