The guide is pretty solid, kudos to Seeed, I've used them for a 4-layer PCB and was very happy with the result.

Even if some things are different across CM's, the guide at least provides an awareness of the main issues for the target audience (amateurs or people getting their feet wet) for which these things are "unknown unknowns." If someone were to follow Seeed's guidelines, they would be in a good position to adapt their stuff for a different CM as long as they're receptive to feedback and the CM is willing to review it.