It is possible that both of you are correct, in that there will be 5 new return trips at times which GO considers "all day".
There are currently 8 southbound trips in the AM peak, arriving at Union between 5:30 and 10:30 AM. The 6 PM trips depart Union from 3:30 to 6:50.
If they add 5 new trips in each direction separated by a couple hours, then they are technically covering the entire day. That said, it does seem quite unlikely seeing as there would be little demand for such a minimal off-peak service. It would make far more sense to run some counter-peak service to Guelph and Kitchener. But that depends on building the second platform at Guelph Central Station so trains can cross. I think I read somewhere that they planned to build it for 2015, but I haven't heard of any work starting yet.
5 round trips does sound like very little given the scale of capacity improvement, but there is still the issue of the bottleneck through Brampton/Georgetown, where GO has to share CN's main line. And don't even get me started on the situation west of Georgetown with the GEXR.