Remember that for a completely-overlapped corner constructed to the theoretical maximum dimensions, the distance between the outermost vertex of the bumper backing and the nearest point on the frame perimeter is:
0.75 in × sqrt(2) = 1.061 in
Since this exceeds the 1 in clause of the rule, that construction is technically illegal.
In fact, the rule doesn't prohibit overlapping backing corners in general (by virtue of not mentioning them at all), it merely prohibits this degree of overlap. To confuse matters, it actually requires teams to fill the corners with soft materials—and this could be read to mean that the entire corner must be soft.
Additionally, the statement "wooden backing may only extend the length of the horizontal projection [of the frame perimeter]" is a bit dubious. The maximum length of a
segment cannot exceed the "maximum horizontal dimension" (unclear whether that's a diagonal across the frame, or a side length), but this doesn't say anything about overlap. Further, although there is a requirement that "a full segment of BUMPER must be placed on each side of the corner", it's not clear whether that means the segment must begin at the vertex, or merely whether each side of the corner must be adjoined by such a segment.