I'm not sure what the "ereader app" is that you refer to. reMarkables come with exactly one app "xochitl" that handles all user interaction. Sure, they let you access PDFs to annotate - but again the primary use case isn't to read them. ePub files are consciously converted into PDFs to support that premise.
KOreader, whilst an excellent reading app, doesn't support annotations. So it's not obvious why reMarkable would consider that an enhancement given the primary device purpose.
My car doesn't have space to walk around, nor refrigerators. That's why I bought it. Same with reMarkable devices. I buy and use them for the content creation functionality. I have a Kindle for the times when I want to swap to "consumer mode"