There is truth in what you say. On a pure hill climb, without any downhills, you either put out the minimal effort required to move forward and upward, or you come to a stop, and that requirement for non-stop sustained effort can be very energy depleting. Whereas flat riding allows you to stop pedaling and just coast, while you recover, or you can put out a very low amount of power and still move forward. In that respect a hill IS much harder, because it REQUIRES constant effort, whereas flat riding allows someone to slack off and recover a bit, or go as slow as they like. I've ridden 31,000 miles in the last ~6 years, and my hardest efforts (1,000+ watt sprints, and a 24 hr ride of 325 miles) were all on the flats. I've also done multiple 6,400' sustained climbs to the top of Mt Evans in CO to an elevation of 14,100', in my granny gear, at a snail's pace, while being severely depleted of oxygen, but going slow, so it was "easy". As well as riding many 1,000's of miles of commuting, sometimes at 500 watts to get some training in while I commute, and a 111 mile gravel ride with 13,500' of climbing. But when I think back on it all, the stuff I did on the flats was the most I've suffered. The gradient of the road in front of you does not dictate how hard the ride will be, that's all up to the rider.