Sorry, but you misunderstood what they wrote.
From the article: “In addition, half of the earnings model is based on a proportional share.”
The proportional share refers to the proportion of a reader’s total reading time spend on a short story, not the proportion of time spent on the total.
Then they wrote: “If you write a short piece that is read by people who predominantly read other short pieces… then your story will receive a meaningful portion of share earnings. The outcome will be similar to if you’d written a long piece read by people who predominantly read other long pieces.”
In other words if readers read both long and short pieces, the proportion of time spent reading the longer article will be greater, leading to greater compensation.