lmao, this is a theory, and this study hasn't only not been peer-reviewed, it doesn't seem to have gained much of any traction in the academic community in the last 20 years.
if I paid 600 college students to fuck each other, yeah, I'm going to find that one penis is going to squish out the cum from the last penis, but that doesn't prove that it evolved specifically for that purpose. isn't simple displacement, and a task that could be as easily performed by a dildo or fist or anything else that you shoved into the jizz-filled cavity.
A theory is the most solid thing science can produce. What you’re wanting to say is hypothesis. Also, OP should provide the relevant section of the original review by Baker and Bellis 1995