No USB cable has “gigabit speed”. It probably has 480 Mbps (USB 2.0 standard).
Maybe he meant a 5 Gbps Gen1 cable. That would be “gigabit speed” but still rather slow by today’s standards and won’t support DP. They are pretty cheap these days, so wouldn’t be suprising to see left over stocks being sold as charging cables.
I meant that no USB standard actually has exactly 1 Gbit/s. I even mention that next one if 5Gbit/s. Just a misunderstanding I think.