Source? Yes. IBM. IBM made crappy clones. I am not saying those are absolutely IBMs, they could easily be clones. But I do recall IBM making a case very similar to that. I also speculated, it doesn't mean I was stating it as fact under oath.
"I know for a fact" - why so accusatory? No, I'm saying those particular machines were sold with 5.25" drives and then upgraded to 3.5" because none of the plastic matches.
"supporting more than one modem at a time is literally the definition of multitasking" - no, it isn't. You might as well claim that a single threaded application that can respond to the UI and also do processing in the background is multitasking when it clearly isn't.
"You can't do it without multitasking" - yes, you can. There is a vast difference between a multitasking OS or multithreaded program and simply polling a couple of devices or getting IRQ'd. Now this I know, "for a fact" and we can argue about that until the cows come home, but frankly, I have more productive things to be doing.
"Sounds like overconfident and incorrect speculation" Take five minutes and just chill. There was an awful lot of speculation in my post, and I was very clear to say "I believe" an awful lot, based on a grainy image, and a hazy memory of wrangling those kinds of BBS setups but apparently reading comprehension isn't your strong suit. This isn't reddit, you don't need to go full bore attack because someone said something on the internet you think might be wrong.