this is still beyond me how EU treats iOS and iPadOS as two different operating systems. iOS was designated gatekeeper at the start of the whole DMA thing, iPadOS was just recently.
What do you mean? EU is considering them as the same from the start. Are you talking about the process that happened because Apple appealed and wanted them separate?
No, the EU considers them separately. Apple did successfully convince them that they are separate, however the EU then said that iPadOS would soon cross the threshold itself so would preemptively include it