This is false, Apps don't have to be programmed in a way to support multitasking.
That's a broken "argument"
Um, yes they do. Apple's own documentation says they do.
https://support.apple.com/en-us/HT202070
The three methods of Multitasking on an iPad is Slide Over, Split View, and Picture in Picture. All three require app support, and many still do not support any of these.
Switching between apps with gestures IS NOT multitasking.