There have been about 30,000 days since he started working in 1930. If the 300,000 number is accurate, he would have had to fix an average of 10 typewriters, every day, for the past 80 years. That's without any weekends or holidays.
I guess I have no direct experience repairing typewriters. However, I would have certainly guessed that it takes longer than ~1 hour to "fix" a typewriter. In addition to that, I would think it's hard to find a stream of that many typewriters to repair. (I.e., a rather successful business.) If these numbers are true, the guy was pretty impressive.