Collateral form can and is often taken into account when assessing a horse's official handicap rating. Once entered into a handicap.
So whilst they won't "purely" handicap based on the subsequent form of others, they will certainly consider it and factor this into calculations
In this particular instance given the protagonists in his 2 maidens being the first and second in the supreme novices and considering the relative distances beaten or won by, i'd expect them to factor that in and not rate him as if time ended after his maiden win.
I would acknowledge that they may allow for some leeway in respect of the others improving as this would be understandable, but you could not argue with them if they did rate him 150+. Which is what I've done.
Like I said, we'll probably never know and we are almost certainly wasting our time here.