The RFC defining them says they're the same and has since the earliest draft I can find, also from 2002. You should offer more explanation when you take a stance contrary to what is well documented.

How so? I thought they are the same, at least almost.

Tremulous (ioquake3 fork) had GUIDs from qkeys.

https://icculus.org/pipermail/quake3/2006-April/000951.html

You can see how qkeys are generated, and essentially a GUID is:

  Cvar_Get("cl_guid", Com_MD5File(QKEY_FILE, 0), CVAR_USERINFO | CVAR_ROM);

So, in this case, GUID is the MD5 hash of the generated qkey file. See "CL_GenerateQKey" for details.

> On startup, the client engine looks for a file called qkey. If it does not exist, 2KiB worth of random binary data is inserted into the qkey file. A MD5 digest is then made of the qkey file and it is inserted into the cl_guid cvar.

UUIDs have RFCs, GUIDs apparently do not, but AFAIK UUIDs are also named GUIDs, so...