I am very interested in an RFID reader that acts as a keyboard like the author describes, but for some reason they don't specify which reader the entire article is about.
Does anyone know, or know of one that acts similarly?
The Chameleon Ultra V2.0 open source project [1] can be configured to "Reader-to-HID" which should give you what you want. You can build your own, or buy one of many pre-built options [2]
[1] https://github.com/RfidResearchGroup/ChameleonUltra/wiki
[2] for example: https://www.aliexpress.com/item/1005009580619682.html
That should be the most common behavior. Barcode readers work the same. In the past, you even plugged barcode scanners between keyboard and PC and they injected the key codes of the detected code into that.
The ones I have tried in the past, at least under Windows, only seem to have support for the PC/SC Smartcard API, so you typically need a specialized application to use them, unless it's just for logging in to Windows itself, but I'm more interested in logistical/asset tracking purposes.