I have yet to see the "FPGA is less power efficient" thing to be true. People are always comparing the same circuit in an FPGA Vs an ASIC but this is a nonsensical comparison because of three reasons:

FPGAs have hard wired blocks like DSPs which do not have any power disadvantages vs "ASIC" (only advantages actually)

the likelihood that there is an ASIC that happens to implement your particular design is very low

and off the shelf ASICs like GPUs and CPUs have significant amounts of overhead for each operation. This is especially evident with CPUs. They perform a small number of operations per cycle, but they have to pay the entire fixed energy cost of caches, registers, instruction decoding, etc per cycle. This is way way worse than programmable logic if you're mostly using the DSP and the block RAM slices.