My theory on this, (could be completely wrong):
Previously, every two stalls shared a single power supply.
Now, all stalls share power source (some combination of transformers and batteries), which has enough to supply most stalls without throttling.
However, I would still think that the speed would go down if all stalls are used. But this should be enough for Tesla to claim that the previous power-sharing no longer applies.
It doesn't make sense for Tesla to reserve capacity that will rarely be used. Most supercharger stations are not full, except for a few urban locations. And even those locations are not full at all hours of the day either. This would be the most economically sound implementation.