I dunno, it seems like it's a particularly virulent respiratory virus. Not a doc either, but the empirical data shows a lot of textbook behavior. The flu is a respiratory virus, and causes diarrhea. When the data infects your body's systems, those systems go haywire, doesn't feel too far fetched to me.
Remember, gain-of-function research is most commonly advocated by ethically-flexible scientists (Fauci) whose dreams of creating world-saving vaccines shut out the possibility of bad things happening. The reason it is so roundly rejected by the non-mad wing of science is specifically because the scenario we've seen unfold with COVID can very easily happen. It's why that type of research is done in China, because they're the only ones who are willing to risk a world-ending event for a couple million bucks. Personally, I don't think it was intentionally released on the world. Probably just an accident, because the average worker in China is not very competent and simply doesn't give a fuck. We don't get poison in our dog food, lead in our kids' toys, etc. because the Chinese workers are malicious, they just don't give a fuck. Oops I spilled arsenic in the baby food vat. Oh well, time for lunch. Probably some lab worker infected himself due to poor practices, decided "fuck it, I feel fine," and went about his day.