Im not wrong!
If all that's true then why does Snapdragon have better battery life? As I said in my comment the great battery life comes from when the CPU isn't being used. It's everything else around it. That's where AMD is still significantly behind.
https://chipsandcheese.com/p/arm-or-x86-isa-doesnt-matter
https://chipsandcheese.com/p/why-x86-doesnt-need-to-die
All instructions across x86 and Arm are being decoded to micro-operations, which are implementation specific. You could have an implementation which prioritizes performance, or an implementation that prioritizes power consumption, regardless of the ISA.
Decoding instructions, particularly on a modern die, doesn’t consume a significant amount of area or power, even for complicated variable length instructions.
No, the main reason for better battery life is the RISC architecture. PC on ARM architecture has the same gains.