If the panel has grid-power, the two waveforms would not match up, and the inverter would go poof.
If the panel does not have grid-power because of a power blackout, you are energizing a line that runs out to the street pole, and you are creating a life-threatening situation to a lineman trying to restore power.
So, never do this without the proper either/or switching breaker that isolates the inverter from the grid.
High freq hybrid inverters are not bi-directional in the same way a LF hybrid inverter is. A LF hybrid inverter can slide between push and pull continuously with a slight tweak to PWM duty cycle. A high freq inverter makes a hard mode switch between push and pull and must be synchronized to applied AC input to run the PWM output bridge as a rectifier for charging.
A LF hybrid inverter cannot stand application of unsynchronized AC applied on ACout either if inverter is already running. It will immediately shut down inverter operation to save itself.
A LF hybrid inverter has the advantage of battery taking any sudden surge back. A HF hybrid inverter has just a HV DC capacitor to absorb any backfeed surge.