That's right, Steve. But it's not so clear cut when the music is more chromatic and the key isn't so well defined or shifts around a bit. Monk mentioned that a guitarist might solo in the Dorian mode over a long vamp in D minor7. I don't remember if he mentioned what that chord is in relation to the key, (e.g. chord ii or vi or whatever) but it doesn't matter, because the operative word is "long". If it's long enough, the root of that chord will start to be heard as a new tonal centre. So in that case the solo really is in the Dorian mode, because it's relating to the new temporary key centre D and the true Dorian mode quality will be free to emerge. But if it's just passing through Dm7, then it won't cause us to lose sight of the original key so there would be no justification in calling the few notes of the solo during that brief Dm7 as belonging to the D Dorian mode.
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