Earlier quoted context omitted.
I can see their thought process— "Netflix will most likely exist even if we don't provide them server services as there exist other cloud providers and / or Netflix is big enough to roll their own, so we might as well have a piece of our competitor's success by having their infra spend come to us."
Netflix absolutely should roll their own system. At the very least as a meta layer on top of the major cloud providers.
That's not to say it might not be a worthwhile approach over the long run but it would be a huge strategic investment in something that has essentially nothing to do with what they view as their core differentiating service to customers.
Would Netflix have done things differently if they had known they'd reach this level of usage? Perhaps. But disentangling now would be very difficult and almost certainly disruptive--including a likely reduction in quality of service.