I was gonna ask how they could legally do this, but I went to try it and... it looks like they just copy the gameplay without using explicit IP assets and just load it on the katamari page, so it's probably legally fine.

I wonder why they added this? Just a google employee having fun? or paid partnership? did something katamari-related release recently?

There's a whole bunch of stuff like this! Looks like this website has been collecting / recreating them: https://elgoog.im/

One of my favorites that I remember was "Zerg Rush" (https://elgoog.im/zergrush/) which was a tribute to StarCraft.