I’m the user who emailed.
I’m very fond of this feature in the self-hosted reader tt-rss, which I’m hoping to get away from. Another nice feature is that each feed heading has its own “mark all as read” button (the double-checkmark in the screenshot).
There’s no paging within each grouped feed. Instead, only a little more than one screen’s worth of content is loaded and ready to view. If you scroll down to the end, after the last item you get a “loading, please wait” message. After a short pause, the message is replaced with the top of another page of content which has been added to the current view, allowing you to resume scrolling down as if it had been there all along.
So, if there’s 500 to see in a particular feed, you’ll have to scroll through them all before you can see the next grouped feed.
It helps that this reader has filters to limit the amount of content you’re looking at. The default selection, instead of “All articles”, is “Fresh articles”, which is unread and just-read items less than x hours old, and the user can configure this number. It is very handy to just see what’s just been posted today, rather than including every post you missed since the last time you looked at the reader.
In any case, grouping by feed isn’t for everyone. When the feature was added to the non-free reader BazQux, they said “Some people prefer to read this way and it actually works quite well if you don’t have too many high volume feeds.”