The bright red corrosion we see there is a sign of over firingCurious what to that guy shows signs of over firing?? Not being snide here I am genuinely curious because most stoves seem to look like that anyway. Mine sure does. As well as the "rusting" most of which would probably clean right off if you actually did attempt to clean it - in which case, its not even rusted! Heck mine starting forming that "rust" inside after it's first year. I'm no pro here but that stove interior looks pretty good to me for being 20 years old.
I got a buddy who bought a very old farm house a couple years ago. It has an early 90s Earth Stove (i forget the model but its one of their imports) and the baffle is very clearly cracked. Smoke seeps out of the back at start up because there's also cracking/separations in the back of the stove. Long story short, he had it inspected and the guy told him it was all A-okay! Signed off on it being safe. I couldn't believe it. The cracks are plain as day! He got it inspected again this year and that guy did not pass it. This is all to say that chimney sweeps around here in NY seem to be one extreme or the other - They either don't care enough to bend down and actually look up into a stove or they will write up 50 different things you have to change/buy from them to make everything safe.