I guess that's inspired by the famous "plan to throw one away (because you will, anyway), from Freed Brooks if I remember correctly.
IMHO it makes sense if Plan B is much simpler, but not as efficient as Plan A (remember, this is a game developer, everything is about efficiency). You build Plan B is a prototype, and could still fall back on it (and iterate on it) if Plan A fails.
If Plan B is more complicated than Plan A, doing it first sounds... dumb.
If the point was "write a prototype/less efficient version prior to the more time consuming but better quality solution" then maybe they should have written that.
The connotation with plan B is you do plan A, then if it doesn't work, you do plan B. If you are supposed to implement plan B first then really plan B is plan A.
IMHO it makes sense if Plan B is much simpler, but not as efficient as Plan A (remember, this is a game developer, everything is about efficiency). You build Plan B is a prototype, and could still fall back on it (and iterate on it) if Plan A fails.
If Plan B is more complicated than Plan A, doing it first sounds... dumb.