Fair enough, but where would you draw the line between a PLA and horizontal microcode?

I would say that the difference is not the outputs, but how the information is stored. Microcode is "software" that is stored in a ROM, so you have one row of the ROM producing the outputs. You can point to one line of the microcode and say "here's the micro-instruction", and change one bit of it easily. (The definition of a ROM having one row active at a time while a PLA can have multiple rows active is from "The Architecture of Microprocessors".)

You could use a PLA to produce exactly the same outputs from the same inputs, but it wouldn't be microcode. The PLA is "just" an optimization. But with a PLA, you can't change a micro-instruction without ripping out some of the logic and replacing it. You have a hardware implementation rather than a software implementation, which is the hallmark of microcode. In other words, changing a control signal value in the 8086's microcode is trivial, but changing a control signal in the 6502 is considerably harder.

As a concrete example, the design of the Apollo Guidance Computer is clearly microcoded, with specific microinstructions. However, the implementation is gate logic that completely obscures the microcode design. (Not a ROM implemented with NOR gates, but highly-optimized logic.) So I consider the implementation of the AGC to not be microcode.

(This is kind of splitting hairs, and I'm not really into subtle linguistic arguments.)