Well, I dunno. Certainly that's one definition. Another might be this: an instrument that has significant acoustic properties. I think this is what Brad is referring to. The sounds a synthesizer makes when struck aren't significant in this sense IMO, because they have no real effect on the sound the instrument makes when you use it musically. The sounds an unplugged solid-body bass makes do, however, affect how it sounds when plugged in. If they didn't, a bass made of, say, Lexan would always and everywhere sound the same as one made of maple or walnut or pine plywood. Without getting into the argument of how much body and neck material and construction affect sound, I don't think many of us would argue that construction and material never have any affect at all. As soon as you concede that a solid-body bass's construction can affect sound, to whatever degree, you concede that the bass has audible acoustic properties, even if only modest ones. If these properties are audible, they're significant in tonal terms. Thus, a solid-body bass would be an acoustic instrument by the definition I gave above.
BTW, I don't know if this has ever been done with a bass, but on Buddy Holly's records, part of his guitar sound came from a mike on the strings of his Strat, picking up purely the acoustic sound of his electric guitar.