I have experienced a dive like this on a flight before. The airplane had some kind of pressure failure so the pilot intentionally dove from about 10k feet to 1k feet in 30 seconds or something crazy. Extremely scary, nothing communicated over the intercom to us until we were in a fully banked 180 degree turn heading back to the airport. Honesty surprised this didn't go viral on tiktok.
Are you sure you are not recollecting the altitudes or the cause of the dive incorrectly? Depressurisation at ~10 000 feet does not really grant an emergency dive; in fact emergency dives for depressurisation usually end somewhere slightly below 10 000 ft, which is very much in the breathable zone. Furthermore, pressurised aircraft usually have an equivalent pressure (cabin altitude) of ~6-8 000 ft, which is not too far away.
I do not now re: FAA, but EASA regulations permit flight in unpressurised aircraft between 10 to 13 000 ft without onboard oxygen for 30 minutes. Onboard oxygen is required beyond 30 minutes if > 10 000 ft and < 13 000 ft or immediately if > 13 000 ft.
