Along with the timing, I’ve dug up some pricing and sales figures that leave me convinced it was Radio Shack, not Apple, that was mainly responsible for bringing floppy drives to personal computer users.
First, keep in mind that everybody shipping a 5.25" floppy drive in '78 and '79 was using the Shugart SA-400, becuase that was the only 5.25" drive that was available. [wptrsflop] (The Apple drive was an SA-390, which is an SA-400 without Shugart’s on-drive interface board; the Apple controller and software dealt with things like driving the individual phases of the stepper motor. This didn’t seem to result in any real cost savings to the consumer: the Apple and TRS-80 drives, without controllers, were exactly same price: $499. [bytead] [rsc79 p.12])
The Apple II was released in June 1977 [wpap] at $1698 for a 16K system [ars]. (A 4K system was only $1298, but did not have enough memory to run DOS.) By mid-1979 this had come down to $1045. [bytead] The Disk II was released in June 1978 at $599 for the controller and drive; that price stayed the same through 1979. [wpd2] [bytead] So total system cost by mid-'79 was around $1650.
The TRS-80 Model I was released in August 1977 [wptrs], two months after the Apple II, and the disk drive system in July 1978 [wptrsflop], one month after the Disk II. I can’t find 1978 pricing, but in the 1979 catalogue a 4K computer was $499, the expansion unit (containing the disk drive controller) with 16K RAM was $448, and the drive $499, for a system cost of about $1450. [rsc79 p.7] [rsc79 p.10] [rsc79 p.12]
The TRS-80 was “top-selling personal computer of the trinity [Apple, TRS-80 and Commodore PET 2001]” and the Apple II “a distant third compared to the other two machines,” according to Ars Technica. The figures bear this out: the Apple II sold 43,200 units through the end of 1979, whereas the Model 1 had sold over 100,000 units by October 1979. The differences in early sales figures are even more dramatic: Apple sold only 8,200 units in 1977 and 1978, whereas Radio Shack sold 55,000 units in 1977 alone. [ars] [wptrs]
Sadly, I don’t have figures for drive sales (as opposed to the sales of system units without drives), but we do know that “[d]emand for Model I drives greatly exceeded supply at first.” [wptrsflop] But I think we can safely assume that in the first couple of years Radio Shack ended up with far more floppy-based systems out in the field than Apple did.
So I’m going to say that the Steven Vaughan-Nichols, and by extension the Command Line Heros podcast, is simply wrong, and it was Radio Shack that gets “the credit for [introducing] the floppy drive to the PC world.” Even if you wanted to make the argument that Apple was more influential in some way, it’s simply negligent not to mention a competing system that came out at the same time, was cheaper, and had far higher sales.