Luis blandon
Someone will end up here like I did so I have to respond to the comment. You are partly correct but Gilbert is saying that it's a COMPLETE (not somewhat complete and you can ignore the lack of polish) demo of a wholly unfinished game. Yes it solves problems, but not to the extent its needed.
To revert to his film analogy... Would kubrick have been able to develop the beyond the infinite sequence in 2001 and used it to properly sell his films tone, originality, and final style? (A vertical slice). Doubtful. But did he probably do test shots of the effects theyd use throughout the film and screen tests with the actors to see if theyd be interesting? Of course. The second is not a waste of resources but a valuable proof of concept.