Relevant quote:

  Index-only scans are opportunistic, in that they take advantage of a pre-existing
  state of affairs where it happens to be possible to elide heap access. However,
  the server doesn't make any particular effort to facilitate index-only scans,
  and it is difficult to recommend a course of action to make index-only scans
  occur more frequently, except to define covering indexes in response to a
  measured need

And:

  Index-only scans are only used when the planner surmises that that will reduce the
  total amount of I/O required, according to its imperfect cost-based modelling. This all
  heavily depends on visibility of tuples, if an index would be used anyway (i.e. how
  selective a predicate is, etc), and if there is actually an index available that could
  be used by an index-only scan in principle.

So yeah, I don't know if your use case is exceptionally lucky or if the documentation is just pessimistic (or both). Good to know you can coerce the query planner into doing an index scan though.