I fully agree with your statement, because Occam's Razor says that's probably right.

But... If the grease in the FWB around the pawls is nasty, it might be sticky, or gritty, which would make it release the pawls slowly. In normal riding where OP remembers to downshift, the cassette will be moving slower when he takes off, giving the pawls the time they need to extend. In the higher gear combo when he forgets to downshift, the cassette will be moving much faster when he puts torque on it, not giving the pawls enough time to extend, causing what OP is describing. It's fairly unlikely, but absolutely possible.

A version of it happened to me a few years ago when it was -2°F outside as I was leaving work on my studded MTB. I got about a mile from my job and stopped at a light. When I got on the pedals to go forward my cranks just spun. I had to walk home about 6 more miles. Put the bike in the garage and went to bed (pushing a bike through deep snow on the sidewalk is a lot harder than riding over compacted snow and ice, so I was tired). When I woke up, the bike was fine. Cassette hooked up instantly and I rode around the neighborhood about an hour, just making sure it worked. I rode it to work with no problems, but the same thing happened leaving work the next morning. I finally figured out the grease on the pawls was getting cold enough it wasn't moving anymore. I replaced it with synthetic grease one grade lower and put the wheel in the freezer while I slept that afternoon. Even after being in the -5°F freezer for 6 hours, it worked great after the grease change. It hasn't slipped on me since, but it also hasn't been below 10°F since that winter.