There are NO S/O's on permanent bases.
Thanks for taking the quote out of context from the rest of the sentence.
FO permanent base slots can be held by any FO, or SO that has been assessed. Unassessed SOs cannot bid for a slot.
You are confusing the difference between holding the permanent base slot, and the physical transfer to the permanent base.
FOs do not need to take up that slot immediately, e.g. they may need to do a type conversion. If CX let them take it up immediately CX would be up for all the hotel and allowances in HKG while they convert.
Likewise an SO would need to complete their upgrade before being allowed to transferred to the base slot they hold. Otherwise CX would be up for all the hotel and allowances from the date of the slot being awarded and them being transferred to the base.
Nothing stopping 20 base slots being held by 20 assessed SO, if they have the seniority and no FOs applied for them.