AC103, you say:
This begs the question what are they 'providing' for.
What they were actually providing for is later clarified in AC 23-19A - Airframe Guide for Certification of Part 23 Airplanes p.26-27
It makes no sense for the FAA to use an AC issued in 2007 to clarify a phrase they removed from the regs in 1965. The phrase "further provided..." was dropped when CAR 3 became Part 23, in favour of:
"(3) VC need not be more than 0.9 VH at sea level"
.....clearly stated in the source regulation. Read in context with the rest of 23.335.a it means that if the chosen Vc is less than 33.√(W/S) it must be at least 0.9 VH. And in anycase "This AC provides information and guidance concerning acceptable means, but not the only means, of complying with [part 23]...Material in this AC is neither mandatory nor regulatory.
AC methods may be freely chosen, or ignored, by an applicant who seeks to demonstrate regulatory compliance".