what does this mean?

An RFP from a Defense entity, on GSA S70 says:

Since this competition is limited to FSS holders, all services and items quoted must be from a valid FSS contract. Quotations may not include off-schedule items or services. The sole exception is for items or services falling within the micro-purchase threshold. Subcontractors may provide services under their own FSS contract or under the FSS contract of the prime contractor. Similarly, on quotations using Contractor Team Arrangements (CTAs), subcontractors to CTA  members may either use their own FSS schedule or the FSS schedule of the CTA Team Member employing them.

Surely it is settled that the government cannot buy anything on GSA schedule unless that thing is on the GSA schedule of the party it has contracted with. That is why CTAs were invented, was it not? Because subcontractors CANNOT sell off their own GSA schedules, the subcontractors do not have a contract with the government? Is this a new teaching?????