That's the way it's usually done, but I remember that when Matt3o was designing the keyset for the Whitefox (dyesub PBT from an undisclosed Chinese manufacturer — not Gateron/EnjoyPBT) he had samples made. Obviously, it may be more complicated here since there will be a lot of colors, but you never know. At least to see what the cyan, pink and other special colors would look like on a few keys. Since it's dyesub, it should be easier to have a few keys done first than with doubleshot keycaps, but I guess that depends on whether the GB organizer considers it worthwile or if he's confident he can sell sets just based on mockups.
Regarding "what is in the mockups will be very much exactly as what will be produced", there is often some variation. Sometimes it's for the best (Hyperfuse looks better in real life than on mockups), sometimes... ask to GMK TA buyers.