No. Macs run macOS, iOS devices run iOS (and now, iPadOS), Apple Watches run watchOS, and Apple TVs run tvOS. They might have shared heritage but they are decidedly not the same platforms; these are not different form factors for the same thing (aside from iPhone/iPod touch and iPad), these are different platforms with different use cases, different capabilities, different sets of available frameworks including what languages are supported both in terms of compiled code and what can run as an interpreter on-device, different kinds of considerations for interaction, different userbases with different expectations.

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.

Bit like Microsoft in 2005. "Cross platform" meaning Windows, Windows Server, Windows Mobile, Windows CE...