The now dead HP3000s were the same. You could run the IRS on a $150K box, with an acre of EMC drives.
In someone's tortured dreams. It was a slightly buffed out S/36. Those of us who worked on real computers (370, 390, ES/9000, etc.) used to get a big kick out of those little departmental computers. I used to really love how IBM said that they could be maintained by department secretaries. Turns out, it was true!
Even as AS/400's grew (as part of the ill-fated attempt to make SAA real), real mainframes grew much, much larger. Lets face it, todays PCs (even the one I am on) are bigger in storage and speed than the Mainframes of the 70's (and 80's).
The AS/400 was always a toy.
In my industry, the AS400 is known as 'Midrange' processor. For Mainframe we use a 2064 Escon/Ficon, a 9672 Z47 model, and a 9672 X37 model, all OSA and escon adapted, running VM/OS390 2.10 in 32 or 64 bit.