AMD stole it from Intel, though.
AMD stole nothing.
Intel granted AMD generous licensing deals early on when the were helping provide extra manufacturing capacity for Intel. AMD eventually had to distance itself from copied x86 microcode, which they did. AMD's is now a completely independent, unique implementation of the x86 instruction set, and has been for years.
Interestingly, with the advent of 64-bit x86 processing, AMD turned the tables and became the leader. They released a 64-bit consumer CPU--the Opteron--onto the market in 2003, before Intel, so Intel was essentially forced by the market to make their 64-bit x86 instruction set "AMD compatible".
Not quite. IBM would never sole-source back then, not wanting to be beholden to any one supplier. So they sourced both Intel and AMD to make the microprocessors for their new PC. Intel licensed the tech to AMD, and they both produced processors for a while. Then Intel tried to pull out of the agreement for the 386, and AMD sued Intel and won. But that left AMD without a clear right to use Intel's microcode anymore, so they clean-roomed a new processor.
No theft ... there was a licensing agreement thtrough the '286 (or the '386, as AMD sees it.).