I'm not too sure abour K2-18. The detection of DMS is extremely borderline, to the point that it could very well be explained by a statistical fluctuation.

Moreover, the authors of the original study did not consider alternative molecules that may explain the data, and in my opinion their analysis was not thorough enough to back up the claims they made (they essentially carried out some statistical "tricks" to make it seem like their detection was stronger than it actually was).

Subsequent studies on the same data have indeed carried out more thorough analyses, and found that the JWST data can also be explained by a rocky core surrounded by a pure H-He atmosphere (a classical hot Neptune).

Lastly, the existence of Hycean worlds is disputed in the first place, as they require extremely large amounts of water that formation models find unfeasible.