If Facebook limits the filtering to engagement, then it isn't the fault of Facebook that political content is engaging. That's just human nature. Disasters, outrage, politics, polarizing topics - these are all popular topics both online and off-line, and apread quickly as town gossip well before Facebook.
It is only when Facebook steps in and says that particular topics need to be exceptions to the filtering rules that apply to everything else, that they make themselves into a political actor.
For instance, let's say that the news feed showed you content based purely on number of likes. If political posts get lots of likes that isn't Facebook's problem. If the same ranking rules apply to all posts (# of likes) then they would remain neutral. As soon as Facebook says "content from x person will have their ranking artificially changed to reduce/increase engagement with it" thereby making an exception to the rule that applies to everything else, they have now become a political actor.