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Momentary flow reversals expose the valves to cold ambient air on the backside, or at least to high temperature differential. If that's the real failure mechanism, even a short exhaust is sufficient to prevent this. That cold back flow creates a lean condition when the cam overlaps cold air enters the cylinder. Lean equals hot. With that even short pipe if anything gets sucked back in it is hot exhaust gas and not cold fresh air.
Will it happen overnight, no but at 200 miles I bet the damage has started happening.
Believe what you want but I've personally burnt a valve in a chevy v8 with quality race parts (my drag truck) running it with no headers. The Chinese metallurgy these parts are made of are questionable at best.
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