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He could have interviewed the junior employees. Perhaps even gave them their offers

Maybe OP hired/interviewed others teammates. In my experience in small teams, when I've interviewed candidates, I knew the salary range for that position.

salary ranges are commonly posted on job listings on AngelList, etc. so he could of saw it there when they were hiring for the position or if they are currently hiring for that position

also glassdoor posts salaries of different positions at companies so he could of saw it there