It's less about avoiding traffic and more about having direct access to station facilities. With the status quo, busses have to leave the highway and do a detour to reach a bus terminal in order to service a station. This detour can easily add an extra 5+ minutes to the travel time of a route. When you have a dedicated transitway that allows busses to reach terminals full steam ahead, whilst it will be slower than just zooming down the freeway, it still not only significantly shortens the time cost of leaving the highway, but also theoretically allows you to build far more stations and could allow us to build a proper orbital rapid transit route.
The big problem with the transitway of course is whether or not building a full transitway is worth the cost. Something similar could be done to what Vancouver does where they have these shoulders that busses can use to quickly get off the highway and make a stop:
View attachment 427531
While this wouldn't be an ideal solution for larger transfer stations like Bramalea, Hwy 407, or RHC, this could be a good solution for all of the infill stations that Metrolinx wants to build along the 407, say at Leslie or Bathurst.
That being said, I'm not completely against fully grade separating it either. Even if traffic levels don't slow down, it allows us to get a head start on building a massive suburban/orbital network that can one day be converted to a light metro for not much additional cost (due to the ROW already existing), and could be used to jump ahead any possible future developments that cause problems.