Understanding USB Differential and Single Ended Impedance Requirements
I think they're talking about two different sets of wires in different generations of the standard.

First sentence refers to SDP, the two Shielded Differential Pairs introduced in USB 3.0 for 5 Gbps throughput.  These are usually shielded twisted pairs. Their \$Z_{diff} &lt; 2 Z_o\$.

Second sentence is talking about Enhanced Superspeed, the 10 Gbps signalling introduced with USB 3.1.  These can be (high-grade) shielded twisted pairs, but they can also be individual micro-coax wires - one each for the direct and inverted signal of each differential pair. In this latter case because there is little or no mutual coupling between the + and - signals, \$Z_{diff} \approx 2 Z_o\$.