First, this is the DFM guide specifically for Seeed. Every factory will have a different set of specs depending on equipment (assembly, test, and inspection, and familiarity of operating personnel), throughout requirements, yield requirements and price points, rework allowance, and so on.
Second, these typically assume for wide tolerances. Operating environment or design may necessitate relaxing some parameters in order to allow others to tighten (aka the tolerance stack up).
Dealing with both cases effectively in volume production is what requires experience beyond the manufacturer provided DFM guide.
Someone who learns this guide inside and out will be able to DFM with Seeed but not necessarily anyone else.
More generally, the DFM guidelines for ceramic RF materials would be wildly different than for Kapton flex connectors or for standard FR4 material.
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.