I wouldn't call anything close to cross-platform if it only works on Apple's products and devices.
Even with other cross-platform offerings like Flutter, React, Electron and so on, getting one codebase to work out of the box on just two platforms can be a challenge. On mobile, there tends to be a need to drop down from the cross-platform stuff to something native for performance reasons — Discord does this for its chat buffer on iOS, Facebook does this for practically everything.
So while people take to their ideological high horses, there'll be people out there who can really benefit from being able to develop for Macs, iPhones, iPads, Apple Watches, and Apple TVs with a single, familiar, and easy yet powerful API to provide a glass of ice water in cross-platform hell.
Did you mean "cross form-factor" or something?
This is not some far flung usage of the term 'cross-platform'.
Nobody would doubt that Windows 10 and Windows Mobile/Windows Phone are/were different platforms. Nobody would imagine a modern Linux distribution and Android were remotely the same thing, despite their shared heritage and the fact that Android can very happily run optimised for pretty much any form factor that an OEM decides.
It's merely because all of these different platforms are made by the same company that even the slightest bit of skepticism creeps into peoples' minds but make no mistake, they are different beasts with different needs. If they were as similar as people think they are, iOS developers wouldn't need something like Project Catalyst to bring their apps to the Mac, they'd already be doing it.