You can see it if you try the configurations. With 1 gigastack module/switch i.e half duplex, if you "show cdp nei", you see all other switches in the stack as neighbors. Traffic from top to bottom of the stack does not traverse the other switches, so no increase in STP hop count.
With 2 gigastacks, full duplex connections, only the next switch is seen as a neighbor and the hop count increases.