In the 2.5 RlsNotes :: Data Definition Language :: Extensions for SQL Permissions :: GRANTED BY clause:
A GRANTED BY or GRANTED AS clause...
A GRANTED BY or AS clause...
The Syntax Pattern is correct, as is README.ddl.txt, point 9 (line 241)
Also in the Tip, GRANTED AS should by replaced with AS (2x)
as well as in New in Firebird 2.5 :: Other New Features :: Other SQL Language Additions and Enhancements :: 5th listitem.