They are lying. If you look elsewhere for high-gain omnidirectional antennas you will see them advertised in the range of 5dBi - 7dBi gain, but not 30dBi gain.
If you look at the Wikipedia page for omnidirectional antenna, they cite an IEEE paper (McDonald, Noel) that gives an analysis of the omnidirectional radiation patterns directivity vs the half-power beamwidth based on the assumption of the omnidirectional pattern shape. They got D = 10log_10(101.5/(HPBW - 0.00272*HPBW^2)). If we assume perfect antenna efficiency then we can plug in G = D = 3dBi, 5dBi, 7dBi, and 30dBi to get the HPBW for each radiation pattern and an idea of what it will look like. You'll notice that the 3dBi HPBW is ~60deg, 5dBi HPBW is ~35deg, and 7dBi HPBW is ~21.5deg... which all look like normal omnidirectional radiation patterns, donut-shaped with reasonable sized donuts. However, the 30dBi HPBW is ~0.102deg, meaning it's radiation pattern looks like a CD instead of a donut. In reality you can't realize a 0.102deg HPBW omnidirectional antenna, but it's a fun exercise to try and visualize it.