It may depend on your riding style. The frame of a western saddle extends well behind the cantle, so a balanced position in it is when you are 'on your pockets'. As long as you have the flexibility to move with the horse, it works just fine - while allowing more of your weight to rest on the saddle.
In an English saddle, that position puts most of your weight in the rear of the saddle frame and causes it to dig into the horse's back. You need to ride either more vertical (dressage-style) or with more weight in your thighs and leaning slightly forward (forward seat). Either one is going to use muscles in your back and thighs that might not be getting used in a western saddle.
Given the saddle design, this guy is not 'wrong':
From a great website: Erwin E. Smith Collection Guide | Collection Guide
But he would be wrong if he was riding in an English saddle.
I know nothing about how you ride. But if you are used to western, 'on your pockets' riding, then riding well in an English saddle will use and stretch muscles that you were not using.
When I started riding at 50, I thought 40 years of jogging meant I was in shape. And I was - for jogging. At 49, I did 1.5 miles in 9:10, which wasn't bad at all. But riding uses a different muscle set, and needs my legs and hip to stretch where they were tight, and to tighten where they were stretched. And I'll be honest with you. Five years later, I can ride my horse for 45 minutes working on things like turns & changing speeds within the same gait...and crawl off feeling every one of my 55 years!