I was not aware that the --clone flag in gh repo fork takes a boolean argument (e.g. --clone=false). The help text doesn't make this clear - it looks like it doesn't take an argument. This should be updated to make it clear that all the following are valid:

gh repo fork foo/bar
gh repo fork foo/bar --clone
gh repo fork foo/bar --clone=true
gh repo fork foo/bar --clone=false