With out of order execution, speculation, and SMT, it's hard to say if an instruction stalling means that the CPU can't process something else; CPUs are complex parallel streams of processing and trying to think of them in linear terms necessarily misses some complexity.
Another factor (also mentioned in the article) is dynamic frequency scaling. Is a core at 100% when it's running at it's running 'flat-out' at its nominal frequency or when it has boosted? The boost clock generally depends on thermals, silicon quality and maybe time - so in that case what do you make 100%? If you go for nominal being 100% then sometimes you're going to be at say 120%.
The most obvious answer: 100% is what the manufacturer claims it capable of handling continuously under the worst supported conditions.
The more difficult answer/question is how to communicate that 'full use' value as well as the current use (possibly greater than full) to software which calculates a usage estimate based on various already existing interfaces. Or if yet another interface (standard, if thinking about that XKCD comic) is needed.
I prefer the NASA approach: 100% is whatever as defined in the spec sheet, and any improvements above that are measured in percentages above 100.
As an example, the SSME was nominally operated at 104.5%, and the newer expendable RS-25Es nominally operate at 111%.[1]
So basically: 100% is the CPU running at base frequency, and anything higher (eg: turboing/boosting, overclocking) should result in even higher percentages above 100. This would be a lot more meaningful than whatever "100% CPU load" means today.
That's mostly what I stated, though with adjustable frequency products like CPUs and GPUs the question of 'what is the base frequency' is also a question. That's why I carefully phrased around manufacturer's claimed continuous operation in the worst environment supported spec. (Though implicitly with adequate cooling operating, not obviously broken cases like a CPU with it's heat sink shaken off.)
In the case of SMT (aka hyperthreading) you'd only know if the CPU can process something else by profiling a different thread that's running on the same core.
