Please look at the actual data on the linked page. Each of the csv files on that page has thousands of predictions 538 made going back as far as 2008. This isn't the raw data they used to make the predictions, these are the actual predications that they made.
Edit:
For example, here are the headers on the file named presidential_elections.csv
Someone1234|6 years ago
wilg|6 years ago
- Web: https://projects.fivethirtyeight.com/2018-midterm-election-f...
- JSON: https://projects.fivethirtyeight.com/2018-midterm-election-f...
You are correct they do not publish the exact math of the forecasts, but you can see how the forecast changed over time.
tweezy|6 years ago
Edit: For example, here are the headers on the file named presidential_elections.csv
year,office,state,district,election_date,forecast_date,forecast_type,party,candidate,projected_voteshare,actual_voteshare,probwin,probwin_outcome
These are the actual forecasts they made for presidential elections going all the way back to 2008.