Linux is the only OS that fully implements the RFC, and thus the only one that's vulnerable. OSX, Windows and FBSD either don't implement it or partially implement it, making them not vulnerable.

Not quite -- the RFC recommended rate limiting challenge ACKs, but it didn't say to do it with a per-system global variable that would leak shared state. From the paper:

> Therefore, the Linux kernel has faithfully implemented this feature by storing the challenge ACK counter in a global variable shared by all TCP connections. This approach, unfortunately, creates an undesirable side channel, as will be elaborated.

I think it's reasonable to call it a Linux bug. You can certainly criticize the RFC as failing to disclose (or more likely, failing to realize) the risk of an insecure implementation with side channels.