I think Haighus is correct. The only counter argument is that markerlights can modify the BS of overwatch and snap shots (which is more or less what the Syphon is doing). Unfortunately in their usual fashion GW did not use a standard term when setting the opponent BS to 1 they just typed it out. Although codex GK was written before snap shots was a thing, so....
Might need a bit of a FAQ to find out their intention