I never did anything for legs either all my life, never needed to, but ever since coming back to riding in 2018, now older and after a long period of almost no exercise, I'm noticing my leg strength in my quadricep area is just not developed much despite how much I ride. They get tired/fatigued too quickly. Hopefully the exercises help, the upper body and core exercises I've been doing have been very helpful so I imagine so will the leg ones.
I too don't do much exercises in a week if I am already riding a lot that week. However some weeks I either can't ride much or just can't get into a goodmindset to ride as hard as I'd like to, so it's at those time I tend to focus more on the exercises.
I also try to jog for atleast 10 minutes twice a week regardless of how much I ride. This seems to help keep up my stamina, as a lot of times my riding doesn't get my heart rate elevated enough for long enough so I try to use jogging as a way to do that.