If you want to pass a struct-type around which has nullable fields, but you have already checked for non-null (like this one) you need to convert to a different struct-type, which doesn't have the nullability on its fields.

Which is exactly what IMO the author should have done. It's actually a reasonable use-case for inheritance:

    #nullable enable

    record SlackEvent
        ( int EventId
        , string Content
        , string? TeamId
        );

    record TeamSlackEvent
        ( int EventId
        , string Content
        , string TeamId
        ) 
        : SlackEvent
            ( EventId : EventId
            , Content : Content
            , TeamId : TeamId
            );