I'm sure some of the more technical folks on this board will chime in, but in a nutshell what they're saying makes sense. The issue is that at this point, the Powerwall can't roll back the solar production partially. It's either on or off. This means that in order to prevent the solar from overloading the Powerwall's capacity, it'd have trouble letting the full 10 kW system run. I also suspect that they're worried the solar inverter will not react quickly enough to the Powerwall's frequency shift and could cause issues if the solar production is higher than the Powerwall's input capacity.
I realize I'm late to the game, but if the PW couldn't do frequency shifting to signal the solar inverters to reduced output, you would have a problem anytime the solar exceeds the house load in a backup situation. Once the PW is full, there is nowhere for the solar generation to go. Either the frequency or voltage limit would trip the solar inverters...