Being interested in a particular family and thinking of improving its coverage on WikiTree, as a courtesy I asked to join the trusted list of an open profile that the manager had created eight years ago but not touched since (though it needs work). Their reply was that if I touch any member of "their" family I must tell them. Advice please: is this kind of warning off within the WikiTree ethos?
No, it's a wiki but https://www.wikitree.com/wiki/Space:Be_Bold_and_Be_Polite
Possibly you should send them a link to the Honor Code and stress the following:
1. We collaborate. When we share ancestors we work together on the same ancestor profiles.
8. We cite sources. Without sources we can't objectively resolve conflicting information.
I would also recommend you to review the Problem with Members page.