Swift is great! The guy who made Rust worked on Swift for 3-ish years, so there's a fair amount of overlap in interest between the two. Those were the two main contenders, and I guess OOP was the deciding factor.
You can do deep hierarchies in Rust, the thing Rust doesn't have is implementation inheritance. Or more precisely said implementation inheritance that relies on anything but the interface (traits can have default methods but they're part of the trait definition, not any implementation).
Yes, most likely they use it for implementation inheritance which is sloppy anyway since it usually violates the Liskov substitution principle and also most OOP languages that have that concept tend to have issues around co- and contra-variance in either function parameter and return types or containers or both.