No, you're right, I don't see open.silverstripe.org mentioned in the guide, only the obscure 'issue' number :-)
But since we're supposed to report fixes, patches and feature requests on open.silverstripe.org, l think that using the ticketnumber is the best way to link the fix to the problem, instead of opening issues on some other location. Every contribution starts with something being broken or desired, so creating a ticket, or responding to an existing ticket would be the first step anyway...
Others are doing this as well - see the following commit as an example:
MINOR: group ShowInMenus and ShowInSearch check boxes. Fixes #6901 [wilr]
referring to http://open.silverstripe.org/ticket/6901