IB obviously does recognizes it, because margin is crazy small. I have 70k account, opened a position on CL with 100 options (40 short, 60 long) and margin for it is 2.200 EUR.. I mean this is like 3.5 %..
Well I am not with IB but they reference a page where their users can figure it out. I cant tell how you arrived at the 3.5% but I am guessing you are not taking the correct values into account. You should check what the margin requirement is for simply being 100 options short in a spread 40/40 and see whether that isnt also 2200 EUR.