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.
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.