A Digital Forensics Investigator collects data from information systems. This collection may happen overtly or covertly. The presenter in the video did not have formal training on being a digital forensics investigator. He was an accountant with interest in IT, and when the opportunity presented itself, he took it and was able to develop his skills on the job.
This is very interesting, as I know many people in IT fields who do not have an IT degree, but rather had an interest in IT and was given the right opportunity to do a job within IT and develop their skills.
There are different pathways to attain a goal. Education and formal training is one way, but on the job training is another. While I value education because it gives you a good starting foundation, doing the actual job gives valuable experiences that education may not give you.