The answer is in the last phrase of the rule governing the foundation boats :





5.1.2 The Foundation boats Taipan 4.9 (with added F16 spi) and Stealth (R) have the special status of "Formula 16 HP foundation boats"; meaning that, despite the fact that they are dispensated for their non compliance, they have become part of the (closed) Formula 16 HP class. This status is permanent; althought it is lost when the boat in question is no longer compliant with their confirmed (class) setup as fixed on the date 1 januari 2002.