No one used "open source" to describe stuff with the source code available until the term came out to describe free software with a marketing friendly name. People who use 'open source' to describe what is proprietary code with source available are often just trying to confuse things. To be clear, he is not doing that, he's just using 'open', which is fair game, I suppose.
Not to mention Open Source (proper) hardly has a monopoly on the "Open" prefix.