iPhone OS 1.0 was released on June 29 2007, while 10.5 was released on 26 October 2007.
There is the possibility that they have used the beta kernel (after al they started working on 10.5 a while before both releases), but I guess that they used the 10.4 kernel which was very stable in the beginning of development (mid 2005 to early 2006, I guess), unlike the 10.5 kernel.