Thanks to SpeedOfLite:

Road Test/Bike Review (1986) Cannondale ST-400

Glad you asked, tcs . I didn't realize that they were almost exactly the same weight.

The 23" bike in the review weighed 23 pounds, 6 ounces, about the same as the Trek 520 from the same year. (According to Google AI, "In 1986, a Trek 520 bicycle weighed approximately 23.2 pounds. This information is from the 1986 Trek Bicycle catalog.")

So the main advantage of the Cannondale would have been the torsional rigidity. As the reviewer points out, you can stand while climbing hills without the wallowing that can happen with similarly heavily loaded steel frames.

The Cannondales were conservatively built with straight-gauge aluminum until the numbered CAD designations showed up, signifying different configurations of butting in the various tubes. My 1995 H200 hybrid is labeled CAD 1, meaning it's built with plain-gauge tubing throughout.