That's not the entire explanation for Ng's use of Octave though.
At the birth of Coursera in 2012, R and Python were already clearly established in the field of data science. R was the dominant open-source language for data science, with Python very close behind (and already gaining ascendancy among folks who identified with "machine learning" rather than "data science"). I remember Matlab/Octave being more associated with academics/students in EE (signal processing, wireless communications, and the like); if you want clear insight into matrix operations, Octave is great.
I think Ng made a very conscious decision at the time to eschew built-in functions and not get distracted by trendy languages - hence the use of Octave to learn how to implement algorithms at the most basic linear-algebra level.
Even at the time his decision was not well understood nor popular - way back then I remember people asking "Why Octave instead of R or Python?"
I think his choice of Octave was really just to avoid using Matlab. I had taken several other courses around the same time, and they all used Matlab. All of the courses had arranged for a free Matlab license for the time the course was active. You also were given the option of buying the Student version for $99. That didn't last long, I haven't seen any courses offering a free license (or even using Matlab for that matter), and to get the Student license you have to be enrolled at a 4 year institution.
In those classes that did use Matlab, there were quite a few people sticking to Octave, though it wasn't 100% compatable. And when I got to Ng's course, I (and I imagine a whole lot of others) were really happy to see he went 100% with Octave. Had he gone the Matlab route, the old course would be pretty worthless now.
We'll never know what happened behind closed doors, but I think Matlab was sponsoring some courses in order to get new people hooked on Matlab and it just didn't pay off.
I've taken 3 MATLAB courses on Coursera this year and they've all included free student access to MATLAB online. Also, a home use license for MATLAB is available for $149.
That's certainly the goal, but in the courses I took, the Octave users always had Octave specific issues they had to work through, so there were some incompatibilities. This was about 10 years ago, so I imagine Octave has made some progress in that area too.
I still think that for learning the math behind ml Matlab still makes the most sense though. It takes the focus off the programming itself and enforces the matrix concepts. Although python is the undisputed king in that regard so unfortunately it makes more sense to teach that
