As an analogy, consider doing this to a person and trying that same excuse.

I suppose if the robot literally cannot path around someone standing still, it might be a valid excuse? But as soon as you're actively moving to block it, it is obviously obstruction.

Judges get to run off common sense, not programmer logic - the point of the law is that public paths ought to be usable by anyone for lawful purposes, and currently "delivery robot" is a lawful purpose.