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They use a different TCP/IP stack which implemented SACK without introducing this bug.

It's a Linux-specific implementation defect, not an intrinsic problem with the TCP SACK wire protocol or spec.

FWIW the FreeBSD bug only affects the RACK tcp algorithm, which isn’t the default (I think).

Correct, its not enabled by default.

That's in Netflix's custom stack that they contributed to freebsd, and not used by default.