While I would automatically agree that the code is almost certainly not "finished", I was thinking that it might have also been a strategic reason for Batman66 not to be playable at expo, which is also along the lines of what you said.

Stern can present a shiny game, and not have to undergo any scrutiny of how the game actually plays. They can present it without having to worry about glitches, or something breaking down. As such, there's no Pinside discussion about a failure, or how similar the play is to Batman TDK.

I find it hard to believe that the cabinet, artwork, ramps, and toys can be that polished, and yet the game was 'unplayable'.

Another excellent point! They don't even have to deal with being compared to the other games revealed! And if the game is in any way similar to Batman TDK, there would likely be more discussion about the other two, given that they are completely new games. The spotlight wouldn't be on Stern.