This is not a clone. They just matched the *outer dimensions* of Whitney, so that they could swap out their old font and not have to remake all their typography.
That's what the parent commenter means by 'They just matched the outer dimensions of Whitney'.
In other words, metrically compatible. To the untrained eye, metrically compatible typefaces all look the same, because they're meant to be swapped between each other. In my view Whitney and Source Sans couldn't be more different. The stroke cuts in Whitney are angled, they're perpendicular in Source Sans. The lowercase 'b', 'e', and 'g' are very different in both fonts.
Except for the ”H”, none of them are particularly similar to Whitney. It would have been more interesting to compare the glyphs to Source Sans. Its ”g” is basically identical to this one.
EDIT: I created such a comparison. See neighboring subthread.
I'd say this is Source Sans. I'm sorry to trivialise Nebula's effort, but this seems much ado about nothing—they warped Source Sans' glyphs to be metrically compatible with Whitney, renamed the result 'Nebula Sans', got a developer and videographer to produce a whole website and mini-documentary on the matter and called it a day.
Scroll down to "Comparison of Nebula Sans versus Whitney SSm" part of the linked page.
I was literally going to complain here that it was a Whitney clone before seeing them mention it on the page.