Is it not reasonable to assume that if there's a TAG here, a question relating to the subject of that TAG is "on topic"?
No, that is an invalid assumption.
If I ask a question about configuring IPv6 on my corporate network and whether my subnetting/address assignment/whatever makes sense, that is on topic.
If I ask a question such as "How do I get IPv6 connectivity at home?" that is off-topic.
Context is required, tags alone do not determine acceptability.
No, it's not reasonable to assume that anything related to a TAG on the site is on-topic.
Have you read the FAQ?
Server Fault is for
system administrators and desktop support professionals, people who manage or maintain computers in a professional capacity.
Programmers and end users will use the same technologies, but with different issues and those will generally be off-topic on ServerFault even though the same tags could apply.
Each site has a target audience and content matter; some are more specific than others. Server Fault is squarely aimed at System Administrators (and related support staff) and topics which include problems they may face while conducting their job (not one or the other; both).
If you want to propose a new site, Area 51 is a SE site exclusively for proposing new SE sites.
If I asked a question and added a tag that doesn't exist then that tag is automatically created. Whether that tag is appropriate of even relevant to the question is a whole other matter.
e.g. There's nothing to prevent me from asking an on topic question and adding a tag such as
Of course I feel quite confident that tag will be edited out of my question rather promptly and eventually deleted but the point is that it will have been created. It does not mean that questions about lawn bowls suddenly become on-topic for SF.
It's worth repeating that tags can be correctly used by both on topic and off topic questions. It's not the tag that determines topicality.