It's a simplification
This boils down (ahem) to the fact that so far as rocks are concerned hydrogen is almost always there because they've been in contact with water during their formation. Lunar rocks are almost all igneous rocks that have crystallised from a melt, so if you find hydrogen in the minerals, you know there was water dissolved in the magma. Here on Earth, dissolved water can account for several percent of magma by volume. As magma cools, the water can either be ejected from the solidifying rock, or it can be incorporated by created hydrous minerals such as hornblende, micas and serpentinite.
What's unusual about lunar rocks is that they are almost entirely made up from anhydrous minerals making it very likely there was no water circulating when they were crystallising. The current results have come to a similar conclusion through a different means.
Lack of water on the Moon is quite significant as it helps support a theory that the Moon began life as an extremely hot object - far hotter than would be normal for its size - suggesting it was created by a massive impact on the Earth. It also helps geologists work out the rate the Moon solidified and perhaps if the lunar interior is still molten. Water dissolved into magma dramatically reduces the melting point of the rock. If there isn't any in the lunar Mantle it makes it highly unlikely that the interior contains any molten rock and that makes it even less likely that there is any ongoing, or even (geologically speaking) recent vulcanism.