Game developers learnt to code somewhere. Many of them in demos and also warez in the beginning. Sega was competitive at the time, VERY much competing SoA/SoE/SoJ. And I just have a sneaking suspicion in this case they were German.
They HAD the code of the game. It's not a first ROM, its a broken rom on purpose! how could you deny that? It's deliberately a broken joke about the bad physics (they even get worse up the level in the chaotix ones). I think the RESET button not working is a joke too but i don't know for sure (why break it on purpose?). Tails/Sonic are in it because they were always in it.
In late 1995 is when this kind of thing stopped happening because the piracy scene stopped doing this kind of thing except in demos... no more SUPER early leaks and so on, or code sharing, and so on. People borrowed source codes, leaked games, because they all joined the industry as friends. That's what leads to things like this.
So ok - the rom is legitimate and by (someone at or connected to) SEGA if you insist - but it's not a serious beta of Chaotix. It is a joke.
But it wouldn't be the first joke sonic game now would it... Sonic Blast anyone?