You are correct, not Light or chemically damaged.
Nonetheless, still damaged
EVERY stamp that has been through the mail is damaged in one form or another, but that's OK.
We all get wrinkled with age.
Your stamp tells a story, when it travelled, and where it came from.
The teeth to me suggests, it has been kicking around in a shoebox, or a plastic container, where it has collected dust on the teeth edge, given the right humidity, that will turn into "rusting"
Treat it well, it has lasted all it's life, till it got to you.
You are its caretaker.