That’s fairly revisionist. Bert Bos was never involved with Opera to the best of my knowledge, and Håkon Wium Lie only joined Opera later. The initial drafts of the CSS1 spec were done in 1994 well before Opera even existed, and the W3C’s history of CSS lists Opera as the third browser to implement at least part of the spec.[1]
[1] https://www.w3.org/Style/CSS20/history.html