You don't have the believe them. Numerous people have found it clear that they are not sharing it with only trusted parties, but were in fact sharing it with everyone.
This is a factual inaccuracy in the statement from O2.
O2's position is clearly that they normally only share mobile numbers with these unspecified "trusted partners". However, this configuration glitch has led to the mobile number being shared with everyone.
I think most people reading the statement would understand that.
This is a factual inaccuracy in the statement from O2.