Y'know, peace and love to my fellow stinky nerds, but someone really needs to sit down and explain to people what exact RCE means. The acronym Remote Code Execution implies code (the beep boop stuff) is remotely (far away) executed (ran on the computer).
Hence, beep boop stuff that is far away runs on a computer.
Historically an RCE is like, your computer is running Soup Goop server, and Soup Goop server fails to properly parse data it receives, allowing specially crafted input to trick Soup Goop server to execute code.
So, you could like, use Python 3.11 to send some dumb slop to a remote computer address and it'll execute your bad stuff.
In this instance (and many other from Steam and malicious mods in general), the idea is that someone operates or possesses a server which, and when a player joins, the video game server syncs data to the newly connected host which pushes a payload to it.
In simple language, and as a hypothetical example, I operate Stinky Minecraft server, and if you join Stinky Minecraft server, my Stinky Minecraft server automatically pushes mods to your computer. Stinky Minecraft server does this because it automatically ensures you're compatible with Stinky Minecraft server and we can all play and have fun. However, one day Stinky Minecraft server says, "you need Goop Texture Pack Mod 0.2.1.1 and ... INFORMATION STEALING MALWARE HEHEHEHEHE" and that malware is automatically pushed to your machine (also with Goop Texture Pack Mod 0.2.1.1).
Things like this doesn't really fit the category of Remote Code Execution because nothing is being executed remotely as a vulnerability, it requires a victim machine to connect to a malicious host which syncs the payload to the machine. This is closer in terminology to arbitrary code loading, malicious plugin loading, remote installation of untrusted code ... something, I don't know, but it's not an RCE.
Overall, this is more or less a fundamental flaw in the design of video game mods because they're not appropriately sandboxed.