'Microcode' is an abstraction. Commonly in the processor's controller, there is a programmable logic device (PLD) that serves as the interpreter between the machine code (and its extensions) and the actual signalling instruction(s) the controller will execute.

This issue is micro-architectural, and has to do with the implementation of branch prediction technology, and speculative execution. These technologies (at least their core components) are hard-wired and built on necessary assumptions that Spectre exploits. This is why Spectre is thus named as such, 'As it will haunt us for a long time'. It theoretically cannot be fully mitigated without replacement.

To compensate for spectre, techniques are being divisive at the microcode and kernel level (which you did correctly assume) to add enough entropy against the branch predictor and the speculative execution circuits to effectively cripple them.

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