Does it mean anything when a value is written as $!$ instead of NA?

The $!$ marking comes from the endpoint definition file and can also be seen in Risteys. It means that someone on the FinnGen clinical team has checked and no ICD code can be used for that endpoint.

For example, E4_ALBINISM has $!$ under HD_ICD_9. This means that in the ICD-9 there is no code that can be used for albinism. This is a bit different than having nothing (empty string) - when there is no entry then either the lookup for a potential code was not done or no code can be used.