It is definitely not "microcoded" - 256-bit operations are just sent in halves to the 128-bit ALUs and combined for the final answer.
Don't get mixed up between "microcoding" and "micro-op" - the latter is something different, slower and which usually requires some kind of transition in the decoders and uop caches to start reading microcoded ops. The latter is the "normal" or "fast" mode for the CPU and just because one instruction turns into two uops (or macro-ops or whatever AMD calls them) doens't mean microcoded.
Don't get mixed up between "microcoding" and "micro-op" - the latter is something different, slower and which usually requires some kind of transition in the decoders and uop caches to start reading microcoded ops. The latter is the "normal" or "fast" mode for the CPU and just because one instruction turns into two uops (or macro-ops or whatever AMD calls them) doens't mean microcoded.