Discussion link doesn't work unfortunately.
Regardless, and I have not seen the eevblog2 video or verified the calculations: If one needs 3kWh to heat up the water (plus the tank) from T1 to T2, and the heat pump uses exactly that - what is the point of the heat pump? A resistive heater will do the exact same thing minus all the expensive moving bits that can go wrong. Are we sure the heat pump is actually working?