When grinding flaxseeds in a coffee grinder, is it safe to add some water inside?
Coffee grinders are designed for dry grinding, not wet grinding. The mention of water in your quote is for a blender, not a grinder. There are also wet-dry grinders, but that's not one.

So, I'm not sure if it's actually dangerous, but it's certainly not designed to have water in it. And either way, since it's not meant for wet grinding, it's not likely to grind wet things well.

Blenders, on the other hand, are designed for liquids, so it makes sense that blending a slurry might work better than something dry.