In general, you cannot push a LIMIT and OFFSET down through a join, since the result is not guaranteed to be the same. (I have no idea what this gem does about that, but maybe it has some sort of 1:1 guarantee through foreign key information, or maybe it just doesn't care about getting too few rows if there are no matching tables in a join.)

The linked blog post seems to indicate that you can satisfy an OFFSET purely using an index, which generally isn't the case. If you have ORDER BY id LIMIT 10 OFFSET 100000, the planner has to fetch 100010 rows in most databases that I know of.