No, he's quite wrong (I couldn't be bothered, but if he's going to be such an arse over it...)
If the engine has a large valve timing overlap, then effective gasflow becomes dependent on the exhaust design - so it would matter. This overlap also make running rough at idle, and often makes the engine hard to cold-start, especially for a hand-crank start. So the sort of engine we're talking about here has very conservative cam profiles. And a 4-stroke with a soft cam really just doesn't care what sort of old plumbing you hang onto it.
If it were a two-stroke, then there would be more risk of exhaust mismatch. Cylinder ported two strokes inherently have large overlaps and highly-tuned motorbikes may be incredibly sensitive to their exhausts. Then he'd have a point.