“A TCP weakness in Linux systems allows ...” vs “A TCP weakness allows ... on Linux systems”
Technically, it's not just Linux that's affected - it's the TCP protocol itself. The “on Linux systems” part is sort of redundant, and only serves to point out that other operating systems are unaffected as they don't implement that part of the RFC.
> 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.