The point is that it is supported, not that others will not work.
What is special about it is that is specs are in total control of Tivo Inc, so that they can control if the spec changes, and if it does, they will know what it is, to be able to keep up on drivers for it. The fact it co-processes wireless networking is a bonus of TiVo going on their own, or a secondary reason.
The reason the Series 3 platform no longer "supports" the non-Tivo adapters, is because TiVo likely saw the Series 3 platform an opportunity make clean break from support of the unreliable market of PC adapters built for the PC market.