90 minutes of extra walking is a lot of time to offset a snack that could be eaten in 1 minute.
I also 100% think that your offset point is a big part of it. 90 minutes walking the dog outside is doubly beneficial because you are not snacking during the time.
I think it's a fair amount. I try hit 10000 steps a day, go to the gym, do occasional runs.
I should be clear. It isn't that I don't think 500 is enough to make a difference. I legit didn't know if it was a lot of extra calories to burn.
I've been walking my kids from the bus stop lately, and my exercise thing is convinced I'm getting about 600 calories in from that. (To be clear, I run to the bus stop, and then walk them back. Two trips, due to different drop off times.) I assumed I also need to do other work on top of that.