I'm guessing they're referring to Linus' ranting on Intel here: https://lore.kernel.org/lkml/CA+55aFwOkH8RH12Dzs=hT3e7eS3Ckz...

The whole IBRS_ALL feature to me very clearly says "Intel is not serious about this, we'll have a ugly hack that will be so expensive that we don't want to enable it by default, because that would look bad in benchmarks".