All this is true, but has no bearing on the common use of language. When a trader refers to Acme corporation as an $N company, he is indicating that $N is its annual revenue, not its market capitalization.
The other common usage that I've seen is to refer to a company as being worth X billion a year. In that usage the a year bit is not dropped, because dropping it would introduce confusion with the first common usage.
I have no horse in this race, but $x billion company has traditionally referred to revenues. The usage of "___ company" to mean valuation is new (last five years), rare among traditional press outlets, and overrepresented among the startup/Silicon Valley crowd. The idiom may be changing in meaning, but if so, it's in the early stages.
