I do remember a similar thing. With a six ply tire, one layer of canvas may be showing through before the tire needs to be replaced. Or something along those lines.
Then again, I don't have a reference to hand and without that reference and some thorough checking I would not be happy taking such an aircraft up too. Furthermore, once the canvas is showing through it's pretty likely most of the profile has been worn off too. That means that wet runway performance, and possibly grass field performance, will be compromised too. So even if the manufacturer (or boss) would say it's OK, I would not take the aircraft up to land at a "limiting" airfield.
If it's just a single spot in an otherwise good tire, then most likely someone landed the aircraft with the brakes engaged. It may be worth checking the creep marks thoroughly in that case.