Apple argued they are separate platforms, and got around the EU requirements with that. So there are no third-party app stores and browser engines on the iPad available yet. Until now, that is.
So basically Apple managed to delay the application of the DMA to iPadOS by 7 months.
They were the same system, but iPadOS has seen quite a few updates that were specific to iPadOS in recent years and didn't get all of the iOS updates. Therefore I would say that they are different systems, even if they stem from the same system similar to how different Linux distributions are different operating systems even though they have lots of similarities.
I would say they're the same system, differing by some #ifdef's.
A linux distro produce different deliverable medias for server, desktop and mobile use, as well as for different architectures. That doesn't mean the OS is not the same.
I think it's the application store that matters, and ipads can run iOS apps just fine with some zooming/scaling issues. There's even a single checkbox you pick before uploading to make your software available to it as well as iOS.
Apple claims, not particularly plausibly, that they are not.
The timing of the split was great. Thankfully the EU proves to be not that dumb.