Really, from what I've heard so far it sounds like the game has as much non-RNG content as Arena, and Arena's 18 main quest dungeons (all big and expansive with a few puzzles) were its only non-RNG content. Is this a correct guess?
I'm not sure. I'm ignoring the main quest and trying to get into hand-made sidequests now, though nothing's too exciting so far.
The Arena comparisons work in as much as the distances in the world are all fake; trying to fly away from a planet is exactly like trying to walk through the wilderness in Arena, in that you're not actually going anywhere, you won't find anything notable, and you've got to use fast travel to move to another actual location.
Arena made much better use of procgen IMO, there's nothing in Starfield like Arena's cities and towns, and all the NPCs in Starfield are non-interactive beyond a few generic barks, unlike Arena townspeople. For Todd, it seems like procgen was just a way to create a lot of big, boring, empty planets so that he could claim the game was vast or whatever. As opposed to Julian LeFay's approach of using it to try and create a fully interactive, believable world.