Earlier quoted context omitted.
> given they hadn't just had to implement the same thing for iOS You've kind of answered this for yourself; iPadOS _is_ iOS. Apple has, in any case, presumably more or less known this was coming for a year or so; they kind of had to make the argument that iPadOS and iOS were not the same thing, I suppose, but it was always a bit far-fetched that the EC would buy that.
It was actually due to the way "gatekeeper" is defined. The EU has now said that although iPads (still) do not meet the criteria, they are being explicitly targeted anyway. My guess is because this is simpler than expanding the criteria to include iPads in some way.
They don't need to expand the criteria. DMA empowers the commission to investigate, and even declare as gatekeepers, products that do not meet the quantitative thresholds on the basis of qualitative assessment.
EU Commissioner Thierry Breton said: “We continue monitoring market developments and will not hesitate to open new investigations should other services below the thresholds present characteristics to be considered important gateways for business users,”. per the commission iPad passes the threshold for business users elevenfold.