I think it's mostly about where the team puts their attention and how to make the best use of the finite production time and budget. Such demos (or games in general) are like fractals, an infinite amount of people can spend an infinite amount of time getting every little detail right and still not cover everything.
E.g. the goal was probably to show off the new Nanite and Lumen rendering features, and probably MetaHuman for the main character faces and facial animations. At some point you have to say "ok, we focus on this stuff, and accept compromises for other things". If the team's goal would have been to create the best crowd simulation or environment interaction ever, they would probably have achieved this, but at the cost of other things.
E.g. the goal was probably to show off the new Nanite and Lumen rendering features, and probably MetaHuman for the main character faces and facial animations. At some point you have to say "ok, we focus on this stuff, and accept compromises for other things". If the team's goal would have been to create the best crowd simulation or environment interaction ever, they would probably have achieved this, but at the cost of other things.