Anecdotal counterpoint:

I have been quite sedentary for several years, since going fully remote.

I had seasons where I would gain wait and lose weight, and my level of activity remained unchanged.

A few years ago, I started paying more attention to my weight, and got to where I was consistently in my ideal weight range. Again, level of activity remained unchanged.

About a year ago, I started indoor cycling (Zwift) quite regularly — about 2.5 hours per week, averaging right in the middle of my target heart rate zone. Diet remained the same. Weight remained mostly unchanged (still in my ideal weight range — I’ve maybe lost 5 or so pounds, but I lost way more weight when only changing my diet).

So — for me — diet has definitely had more impact on my weight than my level of activity.

People tend to adjust their activity to maintain their energy expenditure without consciously thinking about it.

So a 30 minute session in the day might be followed by more time on the sofa later on.