why does it matter- how do you know that minimally symptomatic patients would necessarily take up a large percentage of the enrollment knowing that it was only allowed for part of the trial? You don't and they won't.
Even if it is a non-ignorable proportion, how do you know it would necessarily be unbalanced between the two arms? You don't, it would not likely be.
Even if it were not balanced, how do you know that minimal symptomatic would necessarily be a prognostic factor of overall survival? You don't, and it wasn't
If you multiply the above three answers together, the most likely outcome would be the minimally symptomatic thing is not going to affect the treatement effect on overall survival.
Mind you that it is effect modification but not confounding that one should worry about. And you have been hacking on confounding (via your conspiracy theory on how DNND would apply halabi) which is not terribly relevant.