I once was in a Cessna 206 on a flight plan that took us up to 17,000 ft. You can fly that altitude fine, as long as you have supplemental oxygen (via a cannula in our case).
We took off at about 3AM, and I fell asleep immediately. Woke up somewhere well above 10K, dizzy as hell and realized what I'd done. I put in the cannula and the dizziness left within a few minutes iirc.
Op clarified it was a Cessna 421, so indeed pressurized. When these are ferried they put fuel tanks inside the cabin (these are called ferry tanks unsurprisingly), so they were indeed within arms reach.
10000 feet is quite a lot from a pilot's perspective. In a glider, this is at least 50 minutes assuming a 200fpm minimum sink rate in stable clear air (no thermals), and in an airplane with a far less efficient wing it might be around 10 minutes (a decent rate of 1000fpm).
Primary training is done around 1500-3000 feet above ground, so you're expected to go through your troubleshooting checklist in less than a minute. If the engine failure is on takeoff or approach to land, which is the most common phase of flight for engine failure, you have tens of seconds for troubleshooting because you must leave spare time to get configured for an emergency landing. (CFII in a previous epoch.)
I'm confused what altitude were they at then that they had only 10K feet left? From the article:
>"Because most small light aircraft are unpressurised, it's not advisable to fly above 10,000ft."