The only thing the nebelwerfer is treated like a mortar for is it’s range and template size. The whole ‘counts as a heavy mortar’ throw away is meaningless. Everything else relevant to a heavy mortar is eliminated elsewhere in it’s rules. Other than range and template size, none of the relevant rules are even specific to it being a particular weapon type, they’re relevant to the overall unit type – infantry/artillery/vehicle.
It is artillery and moves as artillery, it’s even specified ‘when being towed, multiple launchers count as light howitzers’. Mortars are infantry and move as infantry team weapons – if it counted as a heavy mortar for movement, it wouldn’t have any towing information.
Same goes for exceptional damage rules – it is artillery so can only take exceptional damage from heavy weapons. Mortars are infantry and take casualties as infantry support team weapons.
It has it’s own ‘to hit’ mechanics and doesn’t range in so it doesn’t even shoot like a heavy mortar.
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