I mean, did they even mentioned RxSwift somewhere?
Come on, it's obvious that Combine was made because the reactive programming was showing efficiency for years and RxSwift was prominent on Apple platforms. Given that impact made by the community it was easy for Apple to implement the same stuff with minor changes in protocol names. I want to stress that experience gathered by RxSwift is essential there. And now, when Combine released and old iOS will die out RxSwift will become less and less important, because native library always have an advantage.
RxSwift is not the first Rx library on Apple's platforms (ReactiveCocoa predates it by many, many years), and while it's widely used and popular, the pattern originated elsewhere. I don't see why Apple needs to mention it?
I think they mentioned before on these forums that Combine is based on the reactive-streams standard, with modifications to better support backpressure. AFAIK that does not require official attribution like many open-source licenses do, but IANAL.
No, and I don't see why they would. Historically, coming late to market with rebranded technology is kind of their whole schtick, and usually doing it better.
Come to think of it, I can't imagine a single time they've ever credited the trail blazers before them. It's always "Here's this, it's magic, praise be"