This was an emailed conversation 16 years ago, so I want to careful to not mis-represent what he told me.
Yes, some wild fish could certainly be anadromous, as could some hatchery fish, but there are no clear indications, as he said. Although I can't say for sure, I would suggest that his observations of the absence of hy ct from the Lower Fraser is less from a study and more from anecdotal evidence
As to how complex this is, here is what he wrote "These fish don't organise themselves into neat and separate categories like we would wish in order to make the picture more clear. Undoubtedly there are lots of situations where fish with different lifestyles are together at the same time and place. Our understanding of all this is pretty skimpy plus we have meddled in the situation by having a hatchery program for CT... Unravelling the mysteries of these fish, in an area as complex as the Fraser, is an interesting but very expensive proposition. As time proceeds methods for sorting out the various stocks and lifestyles are improving. There are techniques for differentiating stocks by analysing DNA , for determining if there has been marine residency by looking for indicator chemicals and a whole host of other neat ways to put the puzzle together. All it takes is lots of time, money and people!! - which these days are in very short supply!!"
If time, money and people were an issue 16 years, it wouldn't be any different today. If some DNA testing of Fraser River coastal cutthroat has been done recently, I am not aware. Several years ago, I was doing some cutthroat sampling for provincial fish tech, but he indicated that there wasn't the budget at that point for the analysis. No cost for my free labour though!
FYI This is a good read
http://www.shim.bc.ca/cutthroat/ct.pdf