"The company is helping to sponsor an anti-human-trafficking event in April"
It also said its nonprofit arm spotlighted groups that worked on the front lines of the issue in 2017, which could be said to be PR campaign but it is stretching the meaning of a PR campaign a bit.
"The lawsuit includes an order form billed to Backpage that’s dated in December 2016."
Which dates before the campaign done after April 2018, and after the nonprofit arm spotlighted groups in 2017.
So the dates did matter as I suspected. Saleforce did not first do a PR campaign and then elected to do business with backpage (as will_brown said above). They first elected to do business with backpage, had a nonprofit arm spotlighted groups a year later and then did the PR campaign after the police seized Backpage. That implies strongly that the PR campaign was a reaction to the police seizure.