Using your definition, then no, breaking a treaty that has been broken by the counter-signatories is not isolationism.
And no, Trump has never advocated that definition of isolationism.
You have to decide I we're better off being part of NATO, even if we spend more... and being part of that organization of countries, or if we're better withdrawing because of economics that are about .00009% of our budget.
It's not about dollars and sense. It's isolationism and it's all part of the carnival act to woo voters.
And it works in certain segments.