DS do not know why they would not be, horseraddish is strong stuff. We human units like it but no self respecting bacteria would. I leave stuff out of the fridge all the time and over night it is always ok.
You are the second person who I have heart uses those two ingredients together. However, my tante (aunt) makes cherry jello with jullienne beets, celery, and sour apples and serves a sour cream horseraddish sauce with it. It is the best.