I suspect it was less "Alpha is not capable of being low power" and more "We didn't have resources to make low-power from-scratch implementation" including the

I suspect it was less "Alpha is not capable of being low power" and more "We didn't have resources to make low-power from-scratch implementation" including the transistor budget driven by available lithography and constrained by Alpha being pure 64bit architecture. Most importantly, outside of packaging the core in SoC-like setup, Digital essentially could only ship one microarchitecture at the time with Alpha.