Agreed, I've contacted CIC via the comments function and asked the question so if I get anything back.The "Sydney address" is also mentioned here: http://www.cic.gc.ca/english/information/applications/spouse.asp
And since this is a big yellow warning, I assume they consider it to be very important. So both guides AND this warning refer us to Sydney whereas the checklist says Mississauga.
I also don't understand why would they change the addresses so often, because it confuses people a lot. It's a major change and I thought they really do it quite rarely.
I've also just checked the document checklist for Common law, I applied via this, and the address still shows as CPC Sydney with the latest FEB 2019 form... very interesting, perhaps it's a mistake on the spousal form, or they've started pushing outland spousal cases via CPC-M.