Evidence does exist, so this statement is baffling. As recently as June content moderators on Facebook were covertly recorded bragging about being politically biased. [1]
The same moderators explicitly claimed the company was "anti-Trump", and said "I think [all of the moderators] do that [delete posts for political reasons]".
Reading into the source (Campaign for Accountability), it's clear why they neglected to mention the evidence -- because they are politically biased themselves. [2]
[1] https://www.realclearpolitics.com/video/2020/06/23/facebook_...
[2] https://en.wikipedia.org/wiki/Campaign_for_Accountability