Should it be possible to place a diagonal filter on a main road?

At #18.04/55.955162/-3.14336 in Edinburgh. This "dog leg" intersection is now transformed to a 4-way intersection, so it's almost a candidate for a diagonal filter. But due to https://github.com/a-b-street/ltn/blob/e5e5735d608c728b679c00dee25b5a4aa4e4df4b/backend/src/neighbourhood.rs#L241, the user has to reclassify as a local road before considering a diagonal filter. This is inconsistent with how we allow main roads to have regular modal filters at a point. What was the reasoning for this?