[FrankJ]

Your theoretical string defines the perimeter, not the actual frame. So the string stretched diagonally between the two corners- See Nathan's picture- define the perimeter that needs a bumper at least 8" from each corner. Like Tristian's picture So you will need a physical frame to back it up.

R22, Fig 4-2 shows an indention with no exterior corners. Not the case here.

Rule seems pretty clear to me. Hopefully Q&A will respond with something that clarifies this.