This is simply wrong.
You could have a Gigabit Ethernet USB 2.0 dongle. It would buffer a bit - receiving/sending data on USB at 480 Mbps, while receiving/sending packets on the Ethernet at 1000 Mbps.
You'd get real world performance of 300+ Mbps...
Network protocols easily handle speed mismatches - otherwise you'd never be able to connect a 100 Mbps line to a 1000 Mbps switch with other 1000 Mbps connections.
The network switch is the device that negotiates the speed. It will have to fake the switch and sync at 1000 and transmit slower. But we are still talking about 1/3 the speed at which the network is designed, which is not acceptable.