From the report in April, TTC is expecting Bloor/Yonge to cause capacity reductions on both Line 1 and Line 2 in 2028 due to passenger congestion on platform levels.
The Ontario Line is pretty far from guaranteed to be finished at that date to relieve that issue, particularly if that time arrives a few years early.
A non-trivial capacity reduction across the entirety of Line 1 and Line 2 could be painful for both Ontario and Canada finances as that hits a lot of people. This seems like relatively cheap insurance and still quite useful even if the Ontario Line does end up finished before that.
Main report:
https://www.toronto.ca/legdocs/mmis/2019/ex/bgrd/backgroundfile-131545.pdf
Appendix (with most of the detail):
https://www.toronto.ca/legdocs/mmis/2019/ex/bgrd/backgroundfile-131525.pdf
If the DRL construction was funded (as TTC intended) in that announcement then I probably would have agreed about deferring Bloor/Yonge for a while depending on the what vendors thought about the DRL schedule.