I have already stated that the flight from US in this case LAX to JAPAN to Singapore covers about 8775.004 Miles based on the amount of distance covered.
It makes sense that a flight with 9500 miles would take longer.
In your own article it says the flight is starting in Newark so that is added distance. They are also taking the Artic route, which although slightly longer saves on fuel for the longer trip because of the headwinds being in the favor of air travel. I personally have taken this flight about 7 times in my life and can attest to the length of the flight.
But even at 9500 miles (in actuality it is 9520.028 Miles not 9500 miles as in the article) if you do some quick math, with a Boeing 777 cruising at 562 mph trying to cover 9520.028 miles should equate to 16.93 hours IFFFF they were going average cruise speed. This Artic Route cruises at a slightly slower speed. so it will take much more time to cover the distance... Much in the same way if you don't floor the gas on your car, you wont run out of fuel as quickly, they do the same thing while in the air on longer trips.
They don't need to do that for the LAX to Singapore flight because its a smaller distance, as they don't need to conserve fuel as much because they have a refueling point to fill back up the fuel. (Japan)
In the article itself you quoted it mentions the variables that make the flight longer are headwinds and routing variables!!! All still within the time frames that are attainable with a Round Earth Model.
By comparison... On a Flat Earth, Because there are no actual distances to scale, my rule of EYE, it would seem that the distance would be 3-4 times the current distance (just using a ruler) From Singapore to LAX on a flat earth map. If we went with a modest 3 times the distance of 8775.00 miles - we get 26325 miles. Divide that by the speed of the Boeing 777 (562 MPH) we get 46.84 hours direct total flight time (Not accounting for taxi to runway two segments, and time to get up to cruising speed after take off, and slowing down before landing) Clearly the Math doesn't add up on a FLAT earth Model.
"Singapore Airlines’ (SIA:SP) daily nonstop from Newark stretches across 9,500 miles and averages about 18 hours via the North Pole, although the flight can last more than 21 hours because of some wind and routing variables. The final departure from Newark for the longest flight will be Nov. 23. The second-longest flight, Singapore’s nonstop from Los Angeles, departed for the final time on Sunday night and landed early Tuesday—Singapore time—after a nearly 17-hour journey across the Pacific."