The issue is not that the pyroprocessing has a bit of contamination of the plutonium. The issue is that most of the fission products are being removed from that, so you are making the task of proliferation much easier, compared to leaving the plutonium in the spent fuel as such, which Canada has been doing so far. The approach that NWMO has taken is that the spent fuel will be as is, placed in casks and inside the geological depository. That's a far better way to address the proliferation risk.
You're trying to compare a really bad process, which is a Purex process, with a bad process, which is pyroprocessing.