… outdated device … this protocol "Threads".
I wouldn’t say outdated. 2021 model is pretty recent, and will work fine as home hub for HomeKit or Matter accessories that use Wi-Fi or Bluetooth or Thread. (It may not work for Thread accessories that are not made to be HomeKit or Matter compatible, but instead made specifically for other home automation ecosystems only, like Google Home and Amazon Alexa.) Matter is the certification and language to make accessories compatible with various ecosystems.
Apple TV would be a home hub automatically, when set up with the same Apple ID account as use to configure the Home app on iPhone or iPad. Any HomeKit home can have just one administrator account. Configuration is not done on the hub device (Apple TV or HomePod), but instead in the Home app.
You can find Apple TV to be a home hub before adding accessories, i.e. before accessory compatibility comes into play.
About HomeKit and the Home app - Apple
Set up your HomePod, HomePod mini, Apple TV, or iPad as a home hub - Apple Support
Check: Home app (e.g. on iPhone)﹥Home tab﹥(⋯) More﹥Home Settings﹥Home Hubs & Bridges﹥[ʜᴏᴍᴇ ʜᴜʙs] Apple TV﹥Connected
If more than one hub device is present in a home, than one will be listed as connected and the others will be listed as standby. It shouldn’t matter which one, if accessories can communicate to one of the hubs, and all the hubs connect to the same home network.
Maybe I'm dealing with a factory defect?
I doubt that, but it is not impossible. Only the Thread antenna in is hardware, afaik. Wi-Fi and Bluetooth is easily tested without HomeKit. The rest of the HomeKit functionality is in software. Just keep your device up-to-date.
