alt Hacker NewsThis is even more baffling because Source Sans is open under SIL which permits free commercial use without restriction.
So if the goal was to save money, wouldn't the smart move be to use Source Sans for $0, instead of paying the designer of Source Sans to make some very minor tweaks to end up with another SIL font?
You are misunderstanding. The Nebula people were previously using the Whitney typeface. The organization that produced/sells Whitney changed ownership and the licensing became prohibitively expensive overnight.
They wanted something that could be a drop-in replacement for Whitney with the same letter sizes and spacing, so that they could put it into all of their products without needing any other redesign.
They couldn't find anything like that available off the shelf, and fonts they found that were close to Whitney in appearance were still also somewhat expensive, so their alternative was to take Source Sans, which has a free license, and make some quite significant but semi-automated modifications to it.
Making changes to just the typeface was presumably cheaper than making changes to all of their other designs.