The point could be made that unless the OP was physically present during each and every one of those 90 day visits over a 10 year span then they have no idea what the owner of that car was told on any of those visits.
It could well be that rust was mentioned verbally to the owner 5, 6, or 9 years ago and the owner either brushed the info off or just plain forgot. Maybe the seller was not as brutally honest as thought or maybe forgetful.
My first thought if someone offered me a car like this for 700 dollars would be, “ok, what’s wrong with it at a scrap metal price”.