I believe Dave is referring to the older Whirlpool dryers made before 1966 which has positive pressure at the lint screen pushing air through it as opposed to the 1966 and later models.
Since I’ve studied the design of my 1963 Whirlpool Imperial dryer, the main reason why it was changed in terms of the design was more to do with cost more than anything else. It took a stamping press to make the stationary bulkhead, another for the perforations in the rear of the drum, the blower bearing/blower housing/blower belt was another area of manufacturing cost, the heating element plenum which required even more stamping processes since it’s two separate pieces, UV bulb, many other things which increased the manufacturing cost.
With the 1966 design, there’s just the bulkhead which only required a single stamping press, a drum, motor with the blower wheel mounted on the back, lint screen plenum, heating element plenum which is one piece (welding done for sure at the factory), other areas which had a lot less complexity.
Some of it had to indeed do with efficiency etc, but more than likely were noise and reliability issues which led Whirlpool to do a complete redesign.