Why can compact frame in road bikes be made in less sizes than the traditional frame?
The claim is pure marketing and does not make any sense at all, unless you only care about standover height. It is true that Giant has smaller selection of sizes than competing manufacturers, and Giant might use the claim to explain why, but even their own models are not consistent with the explanation:


High end Giant models have 6, not 3 frame sizes. This is not that many less than competitors who have 9 or 10.
Giant has a series of non-compact "aero" road frames. These also come in 6, not 9 or 10 sizes.


On the other hand, smaller selection of sizes reduces tooling and inventory costs. If a sponsored athlete requires a bike outside standard selection, it is easier to provide them with custom frame than mass produce the wider selection of sizes.