Test voltages and spacings will be different according to the overvoltage category. For Cat II, 2300 V test voltage will be sufficient, at least according to IEC 1010. But it's not impossible to achieve Cat III 3700 V with addtional insulation. The photo suggests something of this kind. A hypex application note clarifies, that they are considering 6 mm distance in air as necessary, which would in fact refer to Cat III requirements.
Of course component choice will be rather restricted under this conditions, but apparently, they found a way.