I'm not familiar with Mallorca's hydrogeology, but I've done some cenote (cavern) diving in Mexico's Yucatan peninsula. The caves are inland, but sea water infiltrates through the porous and channel-filled rock. The water nearer the surface is rainwater.

The narrow region where freshwater and seawater intermingle is called the halocline, it creates a cool optical distortion which reminds me of the "oil paint" filter in photoshop.

This probably is a stupid question, but wouldn't the salty seawater be lighter than the rain water, and thus be at the surface ?

Salty water is quite literally more dense than fresh water. Average seawater is 1025 kg/m^3, pure water is nominally 1000 kg/m^3.

Somewhat related video: dense brine that doesn't freeze in Arctic sea ice sinks rapidly through liquid seawater below

https://youtu.be/WyWn1XJ9kTE

Oh geez, you're absolutely right, I don't know what I was thinking. I probably needed my morning coffee.

It's OK, stingrays like you aren't expected to know about polar regions and super cold water.

Water can stratify if the layers have different temperature, salinity, or change in density from other causes.

https://en.wikipedia.org/wiki/Halocline

If there is no convection or wave-action to mix the saltwater with the fresh, they can coexist in the same vessel, with very slow diffusion of salt into the fresh layer. If the fresh layer is refreshed by rainfall or fresh outflows of groundwater, the top may always be drinkable, even as the bottom is not.

Fresh water is less dense than salt water, so perhaps fresh water seeped through rocks from above and didn't mix with the sea water.

pretty sure it was inland cave, so not sure why the water should not be drinkable and how is it surprising

they mention the caves where flooded 60000 years ago, which is long time ago to replace all the original sea water with fresh water from other sources

or maybe it's condensated water from walls of cave creating layer on top, though not sure how would that work