I requested my report using the link in the OP. The only place Equifax could have gotten this information, with this granularity, is my employer. It's not just a ballpark number or anything like that; it's detailed salary information for the last 10 years, broken down by category (salary, bonus, equity). For the past year, they have granular information for each pay period, including both gross and net.

It definitely comes from the employer, it has per paycheck info.

My report contains PER PAY PERIOD line items from ~50% of my previous employers.

I can even see where I changed my 401K withholding at one point in time.