“A Citation is a big enough jet to (typically) require two pilots.”
With passengers it may be required to have two pilots, but there’s a pilot with a YouTube Channel called “CitationMax”, and he regularly flies that aircraft solo. With all the tech in the aircraft now, there must have been a warning if there were issues with the cabin pressure. Any pilot worth his salt would have descended below 18K feet where cabin pressure wouldn’t be an issue.
A couple of things are wrong with your post. The CE510 Citation series can be flown single pilot with appropriate type rating. The CE525 Citation seriescan also be flown single pilot with the appropriate type rating. The CE500 type rating covers about a dozen different versions of Citations, including the CE560 that was involved in this accident. It can be flown single pilot with additional training and an endorsement. However, when I renewed the insurance on my CE525 this year, the difference in premium was $100k for one pilot versus two pilots. It is our policy to use two pilots, so this was a no brainer. Second, in rapid decompression, you don oxygen masks. Then you make a descent, if terrain and weather allows.