The procedure you referenced in your question only forwards new messages you are looking for a procedure that will also bring over the oold messages: The are several questions/answers on the gmail help forum.
here is one:How do I transfer all of my emails from one gmail account to another?
the summary of the procedure is :
1) Enable POP on the old account and select for all messages.
2) In the new account in settings/Accounts and import "Add a POP3 mail account you own"
3) A key piece of information is how to log into the account:
POP Server: pop.gmail.com
Uncheck: Leave a copy of retrieved message on the server.
Check: Always use a secure connection (SSL) when retrieving mail.
Uncheck/Check:Label incoming messages – Check it if you want to Label
Uncheck/Check: Archive incoming messages – Check it if you want to archive > messages instead of sending them to the Inbox.