This video(copied with time stamp) presents an approach to solving problem 1665C and I understood the broad approach but I am not able to understand that if we have the sum of left over vertices after considering infection is greater than the number of injection operations remaining we directly say that x is not possible, but shouldn't we also consider the fact that even if we have less injections left, they would all still spread in parallel at any given time?

For example lets say that x = 3 and the sum of count values is 4. And this value comes from two sets comprising of two sibling nodes. Then it is possible to infect all of them in 3 seconds, because once we infect one of them the infection spreads to the sibling. So this is why I have this doubt, maybe I might have misunderstood something in which case I would really appreciate any help