If they showed that gameplay, they definitely wanted to see people's response. If they already thought the gameplay looked terrible and the game had to be postponed anyway, they would have said the game was postponed and not published gameplay they already thought would damage the image of the game going forward.
In any case, yes, of course Halo is not indicative of what the XSX can do, it's just indicative of the fact that 343 is a mess right now.
The reason why most people who are dubious about the state of the XSX though do so not because of Halo, but rather the fact that so far Microsoft showed zero gameplay running on the console except for games that could or did easily run on an Xbox One as well, which will of course be pushing the hardware nowhere near its limits. I'm not saying there is a complete catastrophe going on behind the curtains, but if you combine late devkits/issues with first devkits for several developers + work from home + possible unfinished optimization of some APIs (ray tracing seems to stand out there, between The Medium removing the feature from the store page, and Halo before the delay and Devil May Cry announcing they would be patched in later - while on the same DMC they will be there day on on PS5), and you connect all the dots, I don't think it's insane to suggest more than one AAA next gen game is currently running on XSX below what Microsoft expects, and they are hesitant to show it off until the issue is band-aided as much as possible or they just have no other option anymore.