There are two links in this post, the English one makes the guess, the Dutch one that actually tells more about the case does not mention anything of it.
So, probably they were just running stock Android.

And how does the only proper article of the two linked here even fit tech news? It mostly discusses the muirder case (that it is a likely liquidation, involving Swedes).

I found https://www.bilibili.tv/en, but I am not 100% that this is the site that is refererenced by the the article.

You might be from outside of the Eurozone, and/or very young (I mean this in a respectful way). The European Central Bank et al. decided to have a transitional period in the participating countries.

According to Distrowatch and Wikipedia it is based on Alpine Linux.

The Steam Deck does run Linux right? Generally that means the used drivers are not written by AMD and also do not have an auto-updater from AMD. The deck is supposed to update through it's OS'es package manager and supposedly has the Mesa and Linux Foundation drivers in use.