I'm wondering if that's something else. I'd have to think about what simultaneous AM and PM of the same carrier would do to its spectrum. If the receiver is wide enough, an envelope detector should ignore the PM since it doesn't care about phase. But PM is essentially the same as FM, and that certainly produces sidebands. Here it's narrow PM since the phase doesn't vary more than say, 60 degrees or so and the signal resembles AM with only one set of sidebands.