I generally agree that iOS/iPadOS aren't two different operating systems, but "iPadOS" predates the DMA.

Barely... the iPadOS brand was introduced in 2019, the European Commission proposed the DMA in 2020, and even prior to this there were obvious noises being made in Europe with regards to future regulation. Maybe its coincidence, but the timing still lines up for this being a response to the threat of EU changes.