Krown will not trap moisture. But a hasty cosmetic paint-over job can trap moisture.
Krown rust treatment is a pinkish red in colour. From what I believe I observe, that is paint or similar, not a Krown rust application.
If you have an opportunity, explore with a sharp metal object (skinning knife, screw driver). If you discover painted over rust, use that as a pretext to negotiate the sales price lower. Perhaps a couple of thousand lower.
A rusted exhaust system consists of wear items. Not a biggie.
If you do buy, make a habit of spray-washing under the Xterra every 2 to 4 weeks during the winter months. And then get a Krown treatment during the hot summer months.