Why an isolationist America means no historical NATO

Would NATO still form if the United States remained isolationist after the Second World War?

No; the historical treaty depended on American engagement, leaving separate but aligned imperial and European defence groupings as the likelier alternative.

The historical North Atlantic Treaty cannot be detached from American participation. One of NATO's central purposes was to keep the United States engaged in European security and reduce the need for Europe to rearm independently against the Soviet Union. If Washington remains isolationist, that bargain never forms. An imperial-federation defence system involving Britain and Canada would likely stand alongside a separate European alliance. The two groups could be friendly and closely aligned without becoming a single organisation.