The theory behind this is: at the moment, the new Nimbus Cloak makes you go very very fast. I would argue that the mobility it gives could compete with Flash and that's because it's tied to summoner spell use.
Flash is a powerful spell because of the versatility it provides, that's true. However, taking Heal + Nimbus means you get a 25% movespeed boost as well as another 30% speed AND a heal. Could it be possible that the fact that Flash doesn't provide combat benefits means something else + Nimbus could be used in its place?
Imagine laning against Renekton. I would argue having Exhaust would be much more helpful to fight him than Flash. It's only better if the Exhaust gives you a speed boost as well.
In short, my theory is that it's easier to play without Flash this preseason.
Flash offers versatility. It is very good at avoiding spells like Malphite ult, jumping over walls, and being more mobile as a mage.
However, Nimbus Cloak now offers mobility which is tied to Summoner Spell usage. Consider trading the aforementioned versatility for stronger proficiency in offense, defense, or other forms of mobility. Combat spells support laning, and are also on a lower cooldown.