You say that the DB is in the 5th normal form if it does not have compound PK.
But in the solution tables you still have compound keys (bug_id, assigned_to) and (account_id, product_id). Are these not counts as compound keys?
Yes, the table in this section has a compound key. A table with no compound key is implicitly in fifth normal form, so that would not be a good example of a table that needs to be fixed to satisfy fifth normal form. The first paragraph should be more clear that only a table that needs to be fixed is shown.