A78 is still the most efficient ARM Cortex Design, both on Power and PPA measurements. Like, the A710 offered higher peak performance but overall negligible or no additional efficiency; they sold "additional efficiency" on the A715, but still I haven't seen that at all in any benchmark.
Supposedly, the A720 would be the first time they actually release a A700-series core design that can provide increase performance on the same area size since the A78. I'm not sure how much "power efficient" it will be compared to the A78, specially as I'm sure ARM will leverage the better process nodes of more recent chips to attribute potential power savings.
So, in short, the 8100 is so good because the A78 has been the most power efficient ARM design so far, and as they have been shrinking the process node, has been able to factor the efficiency gains.
Will be interesting to see the upcoming Dimensity 9300, that will use clocked-down Cortex X4, that apparently are more efficient than A700-series at peak performance.