Numbers in the teens are considered to be "normal" numbers for WR's, just like numbers in the 80's. Some historical examples are Lance Allworth (#19 for the Chargers) and Gene Washington (#18 for the Redskins). Numbers in the teens were extremely out of vogue for WR's for a lot of years, but they have had a resurgence lately.
Anyway, playing DB does not change things. It would be the same as Troy Brown (#82) playing DB for the Patriots a few years back. Swain did a decent job against Tennessee backups, though I don't expect to ever see him line up as a DB again. It only happened because of a weird injury situation.