The problem is the foll:
[1] CA wrote a trojan FB app to derive psychographic user data for FB users. This let them determine for example how susceptible you were to misleading or fake news.
[2] They then used FB targeting to target these specific people at scale, pushing extreme fake news such as "Obama moving troops to Texas to ensure 3rd term". This is military grade psyops applied at unprecedented scale.
[3] These people, as they were susceptible to manipulation, would then convert at unusually high rates. He says 5% or higher conversion. Conversion was measured as an action like donating money or signing up for a mailing list.
In this way, the entire democratic process was corrupted. The issue is not that there were dirty tricks in the 2016 election. The issue here is that the existence of FB's app platform allows the detailed psychological profiling of millions of people on scale, and then allows them to be targeted at scale using that profiling. This is a clear and present danger to democracy.
Even with these changes, a rogue app such as CA's would be allowed on the FB platform. FB has no visibility into how app data can be used offline.