This is not accurate depending on the heatpump, most with solar diversion technology have a input for a pulse meter that would be linked to the mains tails.
This allows the heat pump to modulate based on available export at the time.
I have this setup with a Daikin EDLA08E2V3 and it is setup and working as expected so I know this is an option.
Not saying all have this or that people would usually read that far to configure this but it is possible.
And buffering hot water or rooms with excess heat using an ASHP will always be more efficient than using a immersion. Just be sure to set include diverted to yes so it will trigger the ASHP correctly.