I’m not a lawyer, but I do feel it’s important to point out that kobolds are also used by Paizo in Pathfinder, where they have most of the same role and flavor (minus gods and traditional racial animosity), including their physiology. While that’s not necessarily open-and-shut, it does show that they do not specifically protect their iteration of the kobold. Moreover, only a specific list of creatures is actually considered by them to be “product identity,” and does not include the kobold.
Also of note is WotC’s Open Game License, which allows the use of some of their concepts and ideas in the creation of third-party products. You cannot plagiarize their Monster Manual descriptions, nor can you copy their stories or unique characters, nor can you use their artwork, but otherwise, kobolds are fair game as diminuitive greedy lizard people with a penchant for underground dwelling and trapmaking.
Granted, there is likely some nuance that I’ve missed, but at large, kobolds as envisioned by D&D should be permissible so long as you don’t use any art that appears on WotC websites, books or other materials and write your own characters, descriptions and history.
Which is not to say you should change YOUR kobolds to be more in-line with D&D, of course. Nothing wrong with being distinct.