This can be true, or it can be nonsense, depending on what specific loudspeakers you are actually talking about -- how 'smallish' they are and how well they are designed -- and whether they are adequately crossed over to a subwoofer, and what SPL you are commonly subjecting yourself to, and whether each element of your system can handle that SPL without audible distortion
A properly crossed over sub + decent-sized 'bookshelf' speakers on stands can absolutely reproduce the 'full range' -- possibly *more* of the full range, and more accurately, that 4 towers (depending on their frequency response).
It's no different than if you have that loudspeaker in front, and he pans them to left front.
Yes, 'cube'-sized speakers are not going to be able to integrate very smoothly with a true subwoofer. And it will be hard to balance the perceived output if the fronts are substantial and the center/rear/surrounds are tiny.
But substantial 'bookshelves'/monitors that are still flat at 80 Hz , with a -3dB pt at say ~55-60Hz , can do it. I have 5 identical ones all around and, plus a pretty good 15" subwoofer. Within their specified rangethese loudspeakers have very good objective on- and off-axis response (directivity) at a very good price for such level of performance, which is why I chose them. Conceivably they measure better in this regard than many a tower. With careful subwoofer positioning and some EQ I can adjust the setup to sound very well at one listening position (at others, the bass will suffer, but that could be fixed by adding more subs). Coverage of the full frequency range down to 20Hz is NOT an issue. It is NOT the issue I have with drums panned exclusively to one or more surround/rear channels. The issue is aesthetic, the issue is with mixing choice, it is a matter of listener preference and would not be ameliorated by replacing my monitors with towers.