Your summary of the 2018 paper was incorrect. They found it made no difference to people who typed 1 space. People who typed 2 spaces read 2 spaced text 3% fast

Your summary of the 2018 paper was incorrect. They found it made no difference to people who typed 1 space. People who typed 2 spaces read 2 spaced text 3% faster with equal comprehension. And the limitations were significant. The subjects were young and had normal or corrected to normal vision. The task was reading paragraphs in order. And I did not see periods inside sentences mentioned.