Hi Patrick
As per me the SAP DATE format does not match with the DATE format of ANSI SQL 92(HANA uses these standards). The most closely it resembles to is NVARCHAR(8). This could be the reason why it is being used instead of SQL DATE format.
Also I had worked a bit on ABAP for HANA, where the same phenomenon was seen. Its actually required because when the HANA artifacts(Analytic Views, Attribute Views, etc.) are exposed to ABAP layer via Dictionary views(DDIC Views). The Dictionary views will not accept SQL DATE format as input.
To maintain the compatibility with ECC could also be the reason to have it as a NVARCHAR(8).
-Shreepad Patil