Here is a link to download the latest Pacific Salmon Treaty - click on the rectangular blue box to download in PDF format:
https://www.psc.org/about-us/history-purpose/pacific-salmon-treaty/
There is language talking about MSF's on page 55.
I would also look at the definitions of "Interception" on page 2.
There is also language on catch equity, meaning that each nation shall be able to catch it's fair share of the salmon that their rivers produce.
It is possible that the section on MSF's is new to the 2018 treaty and that the US was not thrilled by the BC MSF fishery targeting US produced fish - dunno, but it is a possibility.
There are many reports on the success and implementation of MSF's on the net.
It appears to me that DFO has considered what would need to happen to have a Chinook MSF that meets treaty requirement and thinks it won't occur for "quite some time".
You would first need to mark fish then wait for them to return; ie 4 years or more.
FYI there is also language in the treaty about the US funding a BC MSF "subject to the availability of funds" - HIGHLY doubt if this fits with Mr "America-First".
It is my hope that the above helps with setting expectations.