Oh yeah so 6 years later I actually know some stuff about this haha. Basically a few staff went from Waixing to Ruanxin (the developer for Hitek), probably did a bit more work for Waixing on their GBC games, then to Jungletac after that. That message board poster was one of them.
All this activity was based around Fuzhou which was kinda the birthplace of Chinese console game development, having previously been home to Yanshan Software (the head of YS went over to Waixing as a supervisor on their early stuff)
That leaves that "Handymax" cart as kind of an anomaly though. It's dated 2003 yet the games are way more simplistic and crappy than anything Jungletac or Waixing were doing at the time, so I doubt it had anything to do with that aforementioned team. It really comes off more like someone's first homebrew project than the work of anyone with experience in the industry.
There's also a plug & play system called the TV Play Power by TechnoSource with a similar set of games:
https://www.youtube.com/watch?v=l4FKDd6AQCk /
http://www.amazon.in/Play-Power-Video-Built-Games/dp/B00A2XM6HA The design and name are similar to the first version Intellivision plug & play which was also sold by TechnoSource, although the Intellivision ports on that were developed by Nice Code.
My hunch is that maybe those games were developed by some other external team (I think by 03 Waixing wasn't doing much internal development anyway, they preferred to contract stuff out by that point) who then licensed them out to Jungletac, Waixing and whoever the OEM of the Techno Source one was. (Hell, it's possible Jungletac even
rejected them, considering I've never seen them on an actual Jungletac plug & play)
The Waixing-released versions with better graphics and something resembling music might have been overhauled by the original developers or by Waixing themselves (and/or another subcontractor), given their track record of modifying Nice Code, Inventor and other companies' games when re-releasing them on plug-n-plays.