He's saying there needs to be a balance between
1) the size of the screen, which is feeding based, and
2) the width of the screen in combination with the flow rate compared to the actual water volume of the system.
It could be possible to have too much flow per linear inch of screen width, so there is a limiting factor there.
Take it to the extreme. A 1000 gallon system that is only fed 2 cubes/day. Screen is 6" wide x 4" tall. Flow per guideline is 210 GPH. Let's say it's 250. That's only 1/4 of the system turned over the screen per hour. Compare that to a 100g system and that's 2 turns/hour. On the old guideline, a 100g system running a SM100 used a 20" wide screen @ 700 GPH. That's 7 turns/hour over the screen.
I think the new guideline is appropriate for feeding rate size basis, however there very well may be a factor for turnover rate that has not really been considered, and this affects all types of scrubbers.