Your statement is quite context dependent, and does not apply to most modern residential scenarios. “Over insulating” a flex duct is not going to cause condensation.
Metal duct work in an unconditioned space can cause condensation. Wrapping it in (often thin) fiberglass insulation can increase condensation in certain conditions. Improperly vented attics (and/or air leakage from conditioned space) with metal ducts can cause issues.
