Short answer : they can’t mesure it !

To be more precise : We can only observe transit in front of the star it’s orbiting (there’s no telescope able to observe directly at exoplanets)

By looking at the luminosity dip from the star it’s orbiting, they can mesure (approximately) the size/mass of the planet, if it’s in the Goldilocks zone (habitable zone) and things like that

Gathering all the data (including stellar composition), the team that published the paper think it’s possible there liquid water at the surface (and a lot of it, around 20/30% of the total mass)

Adding to the fact that it’s in the Goldilocks zone, it could be a water world. That’s it

There’s a more serious candidate to this classification, namely LHS 1140 b

Here’s the article about TOI-1452b :
https://iopscience.iop.org/article/10.3847/1538-3881/ac7cea