Is this not just superstition?

I've found that people's thinking for this comes from either your first point about sugar, or from failing to update your expectations when you go from, say, drinking pints that will take a while to metabolize, and then taking shots that are going to hit you hard and fast with the expectation that they work at the same speed.

I've never heard a reason that, say, a shot and a beer would make you more hung over than a shot and another shot distributed in a beer's worth of water (so it lands at the same pace).