That's not entirely true. It can be mitigated, but not in a VM environment where each instance cannot be guaranteed to be safe. So for cloud providers, yeah, there's no mitigation (except turn off HT), but for end users, there are microcode / OS updates available for mitigation.That was a case where they were recommending to temporarily disable HT when the flaw was discovered. However a microcode update was able to fix it later on and HT could be re-enabled without consequence.
In the case of the L1TF flaw every Intel processor on the planet with HT cannot be mitigated with a microcode update or software update. The only way to mitigate the L1TF flaw completely is to disable HT in BIOS forever. HT needs a completely new architecture to fix it and most likely even the newest Intel 9000 series processors are affected.