Previously in similar mods before the move from numerical item IDs to tags, fluid sources had their own numerical ID and were spawnable items ingame. My understanding is that this functionality has drastically changed since then, and while fluids don't have a spawnable item other than their buckets, the fluid 'sources' that are shown in EMI or alternatives are dummy items of sorts. This being said, fluids can still be used in other mods that allow to filter fluids by tags, such as gregtech modern, pipez, laserIO, and other mods.

It would be nice if those dummy fluid 'source' blocks in EMI showed the tags of the fluids as items do when advanced tooltips are shown via F3+H.

Now, I'm not sure if this would be a better thing to request in forge, neoforge, fabric, JEI, here, or whatnot, so I figured as this is my recipe viewer of preference, I'd start with a feature request here.

EMI and JEI are not responsible for adding tags in item tooltips, some other mods do that, so EMI would not add this functionality to fluids. EMI's native way of seeing tags would be looking up the fluid and using the tag screen.