I'd be fine with a multi-column UI. Like maybe a collapsible feed view, with the post/comment filling the screen when the feed view is collapsed. I understand the usefulness of negative space. But Liftoff looks like the developer just forgot that tablets exist and set an arbitrary limit that is reasonable on phones. Holding my phone in front of my tablet with both in landscape, it's maybe a half centimeter wider on either side on the tablet vs the phone.