It sounds like Netflix engineering culture is built a lot around trust. Management trusts that only top engineering talent works there, and the sink-or-swim culture, coupled with performance bonuses and a rising stock price, ensures that the engineers are making the best decisions for the company. It's a lot easier to approve a "build" vs "buy" decision when you know that the interests of the engineers pushing for it are actually aligned with the company business interests.
Contrast this to a company with a large separation of incentives between the stock price and the engineers. In that scenario you end up with a bunch of engineers who are bored and want to prematurely optimize systems because there is no clear personal cost to doing so.
I laughed at this because you can't "repeatedly prove" you're capable of developing your own solutions if you're never allowed to do it in the first place!
The enterprise mantra is: COTS or it doesn't happen.
It's saying your engineers are so much more precious and able than the barbarians outside the gate, that they should write their own test harness/DOM selection library/insert commodity software genre here.
Enterprise tools are well-known to have absolutely horrible user experiences, extremely complicated configuration and architecture, and even worse documentation. I don't blame any manager/PM for thinking, "we can do better" with an internal team.
I've heard this a lot in my career (even academia)