Actually the wheel base on the true Mountain bikes was longer. This was marketed as a urban bike more for commuting.

Didn't need as long of a wheelbase since it wasn't climbing hills off road. If you look at the Geometry of "mountain bikes" the early ones the main triangle is more of touring bike proportions. In the late 80's up the chain stays of mountain bikes shortened some and the top tube got longer allowing for more knee clearance when climbing steep hills.
Compairing this bike with 23 inch seat tube to my 98 M400 Cannondale with a 22 inch seat tube the top tube of the 890 is close to 2 inches shorter.