His understanding was correct. Open source is not the same [1]. It might be an alternative but it is not the same as free software.

[1] https://www.gnu.org/philosophy/open-source-misses-the-point....

Open source was coined by the guys at mozilla to avoid the ambiguity people are trying to create. Open source has a clear and well-defined meaning and it's not defined by whether the source is viewable.