Perhaps carrying your firesteel in your pocket ensures enough abrasion to keep corrosion unnoticeable. Much the same as a (real) silver coin doesn’t seem to tarnish when carried in a pocket but noticeably tarnishes when just sitting in the open.

Shrink tubing would prevent corrosion by excluding oxygen from the surface of the rod the same as nail polish or other coatings, hence no oxidation or in other words… no oxygen = no corrosion.