They expressed "Sorrow", that can mean many things, I suppose it all depends upon your viewpoint or personal interpretation. If they used the particular word that you are looking for in this case ie "Sorry" then what - if anything - would that imply? Guilt? Penitence?
I honestly don't know. Are they avoiding using the word to distance themselves, to avoid blame? If so, what are they to do, say sorry for the actions of others who have not been at Celtic Park for a long time? I'm sure the Kellys and the Whites would not take too kindly to people apologising on their behalf and making tacit admissions that those families were involved in a cover up. That's when other lawyers will get involved.
**** the Kellys and the Whites and the rest of them, publish and be damned would be most people's attitude, and I agree, but then I'm not a lawyer and I'm not party to the professional legal opinion being given to Celtic FC on what they can say and what they should never say. Maybe they were told to avoid the word "Sorry", who knows?