Actually that is not true. If the slot is wider there will be no full contact between driver blade and screw, but there will be two shorter points of contact, since these are parallel to the axis of the blade they not tend to lift the tool, which is good. In addition there will be some friction between the tip of the blade, and the bottom of the slot. As a result, he screw will be driven properly.
Compare to a wedge shaped (aka flared) driver. In theory there are always two thin strips of contact, which are as wide as the blade. The down side is that the blade tends to cam out because there the surface asre not paralled are inclined, and there is no friction contact to the bottom.
Parallel tip drivers are also somehow common in general mechanic, from small to big sizes. These are usually considered better, and are more expensive.
In the comments of this article https://www.cnccookbook.com/8-brands-to-consider-for-the-worlds-best-screwdriver you can read about paralled, plus JIS and Pozi cross types, which are unknown to many.