That is a much better approach. Nice work! Couple improvements could be made, mostly because we haven't updated the example iRule in that article (sorry). First, the key and tbl local variables should carry over from CLIENT_ACCEPTED to CLIENT_CLOSED, so there's no need to compute them again there. Second, to help protect against the issue you originally encountered, I'd delete the table entry in CLIENT_CLOSED before canceling the timer. Finally, if you have a critical need to never exceed the limit even by a little, then you should do the table set before getting the key count and then delete the entry you just added if you are over limit and the reject the connection.
Nimbostratus