Return of the Obra Dinn is a pretty short game, I put 13 hours into it by playing quite slowly (and there's a not inconsiderable amount of AFK time in there as well), I doubt I'll touch it again any time soon (because the structure gives it zero replayability value), and I would count it as one of the absolute best games I've ever played. Portal 1 is a 4 or 5 hour game and, again, one of the best ever.

On the other end of the spectrum, games like Minecraft, Oxygen Not Included or Factorio are enormous, all-absorbing time sinks that will consume your every waking hour for months or years on end if you let them.

How do I compare ONI and Obra Dinn and say that either is, in any objective sense, better than the other?

For an MMO, you're engaged socially (presumably in a guild, rather than soloing). In a mobile grind game, you spend hours constantly checking on progress for that endorphine hit.