A Broker with a brokers license does not lease on vehicles unless that vehicle has their own authority and insurance. A licensed broker won't load a truck until they have the truckers authority and insurance in hand and lists them as an approved carrier. As far as Sprinters and C/Vans you don't need authority under 10k just insurance. Most will not take insurance certificates from the trucker, they must be forwarded from the insurance carrier listing the broker on the certificate.
You mention someone on this site. There must be something someone missed, I don't believe Lawrence would let people advertise if there was something amiss.
I might want to change my statement somewhat. A broker does not lease a carrier, they use a carrier to haul the brokers freight.
They can enter an agreement with the carrier to act as an agent for that carrier but that's a differant animal.