I'm unclear how, with no comprehension of how it is done, you are so sure that the method differs from those offered above.

Without seeing the files for that particular skirt I can not tell you which method or methods that artist has used.

Can the skirt model contain hip, buttock and thigh bones? Yes certainly, does not mean that any geometry is assigned to them. They'll be there to aid the control, which then depends on how it has all been set up.

So again, may I please ask, what version of Poser are you using and how best may I help you?