Unless the BESS output is constrained by a PCS. Typically the PCS will monitor the input to the panel from the grid and the PV system and can be set to restrict the addition of any BESS output to keep the total under a specific amperage. Then the total input to the panel is not the full output of the PV system and BESS.
Hmmm....
First, so far none of the PCS systems I've worked with allow you to do that. They all monitor the system output with CTs at a certain point and must limit the output at that point. They cannot monitor the input to a loadside busbar while also monitoring the grid iport at the same time and make the latter part of PCS operations. I don't see why it isn't possible to do what you describe, but so far it does not seem 'typical' to measure grid imports as part of PCS operation.
Second, thinking like an overly conservative member of a CMP, you could still have a scenario where the system is configured to measure the house load and output battery to cover it, and the house load goes above the busbar rating, and the battery, PV, and utility are all feeding more into the busbar (from opposite ends, to be sure) than it's rated for. This has be prevented either with sufficient busbar rating or PCS. It's the same as the situation we've always had with just solar, just taken to another level of extreme with ESS. But this point is all this is still the case without having to do with the ESS exporting to the grid.