This planet is 2.6x larger than Earth and has concentrations of DMS "thousands of times stronger than the levels on Earth".

It would take a lot of cometary impacts to seed the entire ocean with that much.

From the paper [1]:

> Therefore, sustaining DMS and/or DMDS at over 10–1000 ppm concentrations in a steady state in the atmosphere of K2-18 b would be implausible without a significant biogenic flux. Moreover, the abiotic photochemical production of DMS in the above experiments requires an even greater abundance of H2S as the ultimate source of sulfur—a molecule that we do not detect

[1] https://iopscience.iop.org/article/10.3847/2041-8213/adc1c8/...