They weren't at 801mph airspeed. The article didn't make it clear exactly, but:

"The Boeing 747-436 reached speeds of 825 mph (1,327 km/h) as it rode a jet stream accelerated by Storm Ciara."

Ie max ground speed 1327km/h. And:

"The jet stream reached speeds of 260 mph (418 km/h) on Sunday morning, according to BBC Weather."

So if their max ground speed coincides with max wind speed and the wind was perfectly tail, their air speed would have been closer to 909km/hour.

The article says it's "slower than 801 mph", but sadly doesn't specify how much slower. The ground speed was 825 mph, so if Ciara is so impressive, I'd expect it to give a bigger boost than a measly 24 mph. The article doesn't specify the unassisted airspeed, unfortunately. Considering the jet stream was moving at 260 mph, naive math would suggest the air speed was 565 mph.

As an aside, the speed of sound varies depending on pressure and temperature. At the cruising height of an airliner it's typically significantly less than the speed at 0m/20 degrees. In fact at 35,000 feet it's usually closer to 660 mph.

When you get close to the speed of sound in air, all sorts of weird aerodynamic effects happen. Because it varies, the term "mach" is used to denote 1x the speed of sound (regardless of what the pressure or temperature is) - so an aircraft flying at mach 0.85 is at 85% the speed of sound for that height and temperature.

The only thing that matters to an aircraft at altitude is the airspeed.