This doesn't mean anything as far as SS is concerned. There's no flavor, background, or anything in SS - it's a toolkit, not a treatise on how demons advance in powers.
SS is a D&D book, so it
de facto affects Greyhawk and its cosmology. Even then, it affects the implied setting of D&D in many ways. It *is* a treatsie on how demons advance, or at least how Vrocks advance.
It gives the DM something to work with regardless of setting, and if they had left something out just because "that's not how in works in Greyhawk" then they are doing a disservice to DMs that are doing something different.
No one was doing it different until SS came out. They *could* have left the Vrocks out of it, and let you make your own class for them if you wanted to do it that way, like they did with most monsters. They *could* have made a cool class that progresses from Dretch on up to Vrocks. They *could* have at least had a note saying "this isn't how it works by defualt in D&D, but if you want to do it this way, here it is." They did none of those things, so it's now the default way Outsiders advance in 3e. It's not a variant anymore.
It is, as someone else mentioned, like finding out Elminster died by having a reference to his funeral, but no explanation as to how or why it happened.
They have Vrocks in the MM. Isn't that forcing the D&D cosmology (or at least the creatures thereof) on other settings even more than detailing how they advance? At least the former leaves more up to how an individual DM uses them! How can you argue that
not detailing Outsider advancement would be more restrictive the DMs, cosmology-wise, when they give instructions how to do it yourself if you want to, anyway?
If they'd have left it out of the class section, individual DMs would be free to assume whatever they wanted about Outsider advancemnt.
The DMs that are sticking with the way things have worked in the past are free to ignore the demon classes. There's no change to the old flavor, because there's no new flavor presented. Just new rules options. As always, it's up to the DM to decide what does and does not belong in his/her campaign.
Technically, everything is optional if you Rule 0 it. But this wasn't a blue-book DM guide. It was a tan-book player guide. Everything in there is now default turned on, unless a DM specifically turns it off in his campaign world.
But also, see The Manual of the Planes. The great wheel is given as an example of what you can do with the book, but completely different cosmologies are also presented and detailed. The great wheel is no longer the defacto standard. Forgotten Realms, for example, has its own seperate cosmology now.
The Manual of the Planes details the Great Wheel as the default D&D cosmology. It even calls it the "D&D cosmology" throughout and specifically says it's the setup for Oerth the "core D&D world." So yeah, other worlds have different cosmologies, but D&D's default is the Great Wheel. I mean, look at adventures like Bastion of Broken Souls or the recent articles in Dragon. The definitly take place in the Great Wheel.