When the W960 was launched (and all the other UIQ handsets) Symbian was owned by a number of partners but Nokia bought them all out so that they could donate the OS and their (in my opinion, as a UIQ user, crappy) S60 UI to the Symbian Foundation. UIQ supposedly transfered their code to the Symbian Foundation as well but it seems to have been buried and left for dead (certainly for the next couple of SF^ releases any way)

Not sure what is written in the article is true: "the very first Sony Ericsson handset to feature the Nokia owned mobile phone OS Symbian" - the W960 used the Symbian OS