Just for clarification, if they did use a belt which had been used on carbon steel, then used that belt on stainless, it is indeed possible that they contaminated the oxide layer on the stainless steel, making corrosion of the stainless blade possible. It is a known issue when processing both carbon steel and stainless steel in the same shop.
Stainless steel is stainless because of the adherent oxide layer. If you contaminate that layer with bits of non-stainless steel, corrosion of the steel is possible.
So, the supplied explanation could have been actually what happened.