As it says in the RailsCast you reference, you should verify by checking the SQL that is being run via the logs. Before the counter cache you should get a SQL COUNT
query something like this:
SELECT count(*) AS count_all FROM "comment_threads" WHERE ("comment_threads".commentable_id = 61)
and after you should not see one, and instead only see the CommunityTopic loading:
SELECT * FROM "comment_threads" WHERE id = 61