They were licensing Whitney SSm, they switched to a Source Sans derived typeface to save those costs but forked and adjusted it to more closely match Whitney SSm to minimize brand disruption.

In the documentary video, their CCO explained if they used a different existing font they would have had to rework a lot of their media for the different font metrics, while making their own allowed them to have a drop-in replacement.

Because those fonts have different metrics, so they wouldn't have been drop-in replacements, they would have had to re-do all of their branded media.

Loose programming analogy: The free replacements had a slightly different "API" (metrics) that would have required adjusting hundreds or thousands of "call sites" (media using the font), so instead they made their own "API-compatible" (metrically compatible) replacement that dropped straight in.

This is all covered in the documentary short about the font.