They DO have it on their boards. On some of them anyhow. They do NOT have it on all of them. None of the board partners incorporates dualBIOS on ALL of their boards, which is why he said to be sure. EVERY specific board model, especially if it is a lower quality board where they likely may have cut some corners and left some features off in order to make the board affordable, needs to be verified before just saying "well, it's a Gigabyte board from this gen, so it has it", because that won't always be true.
At least as recently as Kaby lake, it wasn't. For Coffee Lake, it remains to be seen because they haven't even released the business or budget chipsets yet, but I'd be willing to bet there are at least a few Z370 boards at the bottom of the pile without it. In fact, I'm pretty much 100% positive that the Gigabyte Z370p-D3 does not have dual bios, and I'm sure there are others as well.