As a layperson I thought that was odd choice too and would be an Earth-centric measurement, but the paper on which this article is based references this paper: https://iopscience.iop.org/article/10.3847/1538-3881/ac51d2

Where they spell out a formula for the speed of sound in this type of planet given some assumptions. It's beyond my understanding, but I thought it was pretty cool. Section 4.4 if you want to have a look.

Then back to the article paper: https://www.aanda.org/articles/aa/full_html/2025/01/aa50438-...

> Using their formula we obtain sound speeds of the order of 3 km s−1 for WASP-127 b.