It's possible that automation worked to decrease labor share in that period, but other factors(communism, GI bill, etc...) had a stronger influence in the other direction.
However, the argument for a current/recent causative link between the two is not a "preconceived notion" but rather a reasoned hypothesis arrived at based on observation of trends and application of logic to economic theories.
It is more reasonable to suppose that:
overall, automation has had a role reducing the labor share (which has been going down despite a couple of brief spikes in the 50s and 60s)
than to suppose:
overall, automation has been increasing the labor share, but this is overshadowed by other causes which have been decreasing labor share.
Thus, if you want to argue for the later supposition, the burden of proof is more heavily on your shoulders to explain the other causes which somehow outweigh the effects of automation on the economy.