They point out in the article that one of the sticking points is that modern aircraft actually can't do that. This test flight will only have a few passengers and a few bags.

They're mostly testing the physiological effects of long flights to determine if it is even worth developing/buying the planes that would be capable of a full load flight at 20 hours.

The jet lag angle in the article was really odd to me. Impacts on the crew, blood clots mentioned in the comments here all make sense. Jet lag though shouldn't be any different on a 20 hour flight than it is going the same distance with a 3 hour layover.

It's still pretty different. With the quick stop, you're still descending, experience the sea level pressure, having to wake up, walk around, change planes, experience sunlight, etc. It starts readjusting your circadian rhythm.

Seems like a very flawed test then. Flights with just a few passengers are quite comfortable compared to fully booked flights.

True but if even this flight is unbearable there is no point in moving forward.