Additionally I tend to pace when I think and this means I can stay at my desk while puzzling something out. The fact that I can burn a thousand calories on an extended day of focus is just icing on the cake.
I see my treadmill desk as something that helps me work, not a substitute for the gym. Maybe dial back the judgement and "harsh wording" in the future, or at least talk to people before developing and/or sharing such a strong negative opinion.
Nothing builds strength, especially back strength, like lifting heavy things. If you have any space at home, a squat rack (if needed to save space, free standing on each side) and a bar with some weight. Just do something like 5x5 stronglifts app and some stretching. An hour (total, for warming up, stretching, and lifting) three times a week and your world could change for the better. Mine did. You could even do this watching nor listening to some entertainment. I had back pain. Hurt to downhill on a mountain bike. Sleeping got worse. Sitting got worse. Then I started lifting and stretching (was stretching for the cycling already, but needed some new stretches for the lifting). As long as I stretch my glutes/hip and pick up heavy things, I have no back issues. The stretching is key to keep the new muscle loose.
The reason I chose the harsh wording is because there truly is no substitute for getting in the gym to correct posture, build strength, build endurance, and prevent injuries long-term among other things.
You just have to make time for it and do the work, just like with anything else worth having in life.