Similar in form factor and basic capabilities, sure. But. And it's a big one. The reMarkable tablets are aimed for content creation rather than consumption. Nobody realistically buys a reMarkable device when Kindles, Kobos, etc. all perform that task much better with all the connected ebook stores, whatever DRM keeps the publishers and authors happy, etc. etc. That's just not reMarkable's core business and likely never will be.
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"
Well, nonetheless, those missing features and a thousand other papercuts led me to send it back and upgrade to a Supernote. It’s not perfect, but hasn’t deliberately kneecapped itself with some purity vision that just makes it frustrating to use. And honestly, I’d rather carry a Kobo and a paper notebook than a Kobo and a digital notebook that could’ve been an ereader, too, except “LOL we don’t make ereaders”.