It may also depend on how you are riding on the flats, and the amount of wind. I do majority of riding on flat roads into a head wind where riding “fast” usually feels the same as climbing. It is not unusual to be putting out 110% power in the drops, and only doing 15-17mph. The big difference for me - not grinding uphill, power near ftp - is that sweat evaporates at 20+mph versus climbing where the sweat drips down face, into sunglasses, and onto bike. Of course on steeper pitches at my weight and low W/kg it is completely different grinding at 3-5mph. Another clydesdale I ride with jokes about his Garmin going into auto-pause when it gets steep and he can’t put out enough power. But a nice 4-6% grade at 80-90rpm? In my experience is actually quite similar to ‘working hard’ on a flat road.
With all due respect to the CyclingTips article, I’m in a club where most people live near the foothills and love to climb. They hate riding on flats, and hate riding into the wind. They hate coming out to our flat and windy rides. If you don’t train on the flats then you won’t be good at it. And I’ve seen plenty of people get defeated by the wind. As a heavier rider it is pretty common to watch people with higher W/kg climb right past me, but on the flats I drop them on long hard pushes, or I turn in better Merckx class 10 mile time trials (Merckx = no aerobars, no skinsuit, no teardrops helmet, no disk wheels, etc).
From where I sit there is a good bit of psychology involved too. Don’t be too quick to write it off to other stuff.
edit: p.s. I found the comment section of the CyclingTips article to be better than the article itself. Like this one:
It’s my understanding from measurements of actual crank velocity that the differences exist between climbing and flat terrain but are insufficient to explain a large difference in climbing vs flat terrain power output.
I recall some measurements made by Eric Lin several years back which plotted the instantaneous crank velocity for each scenario. The difference between highest and lowest crank velocity during a pedal stroke was double on a climb than the flat but relative to average crank velocity these differences were pretty small fry.
In general, people simply need to learn to get the power down on the flats, it takes a higher level of concentration than on a climb where the feedback for dropping power output is more instantaneous than on flat terrain (due partly to the difference in kinetic energy but primarily because of the significant difference in relative resistance forces that dominate in each scenario and their effect on speed - as Raoul says - you slow down faster on a climb than on the flat).
It just takes practice. Lots of people are crystal crankers on flatter terrain.
Bike position can also play a role.