What does

     do not preserve

mean?

The file format has such a field, regardless.

Obviously you set them to some value.

So why doesn't it say

    set them to <some value>

What is it set to?

Even in the code, it is not clear.  Is that because the header is
initialized to zero??

Well, zero is also known as root, which is a pretty damn powerful
uid on the extraction side.  Especially if file modes happen to arrive
alongside, and have not been modified.  Maybe even assisted by "mode preserve"
options.  So it seems dangerous to give people a thing which is written
as "do not preserve uid and gid" but it pushes the modes unchanged!!

I am trying to point out mostly that extractor has no idea that the
value "has not been preserved".  It must operate on the value which was
it reserves, and there is no known concept of "the archive creator
muddled with it".  So this description of "Do not preserve" seems
meaningful to only the creator, not to the extractor.  That is at most
one third of the equation, so the wording seems unsatisfactory.

I'm not a fan of any of this.  If really you care, you make a copy,
chown it, and then run tar.  That is what people have been doing
forever.