At the moment, looking at RTT for the 8th of January the current Manchester-North Wales loco-hauled diagram is still listed diesel loco, and there are three Holyhead-Cardiff loco-hauled services (0534, 1134, 1651) and their reverse workings. That would be three diagrams' worth of services.
On that basis, I'd guess that both Mark 3 rakes are to be withdrawn in January and replaced with Mark 4 rakes, with the third rake joining in on the Cardiff-Holyhead services. In that case, no 175s would be subbed in for the current Manchester service (except for failures). That would fit entirely with the contents of the press release.