I'm fairly certain you are correct. I remember the MajorBBS could handle multiple lines on its own.
I knew a couple of local DOS BBSes that ran multiple lines with PCBoard under DESQview.
MajorBBS could handle multiple lines on its own, but you had to handle ALL of the lines with one box. That meant a serial port interface like DigiBoard which provided some number (8 or 16 or more) of serial ports that you would connect to modems.
For sure. I knew people who ran multi-line BBS's on DOS PCs under DESQview, just like that (running Searchlight BBS, in my case). I know of a four line that was just using multiple external modems and non-standard IRQ's for COM3 and COM4 (since, by default, COM1/3 and COM2/4 share an IRQ).