This definitely isn't true. We have seen microcode updates that work around buggy CPUs before. Often by taking a slow path for buggy cases. For example there were microcode updates to "fix" (moderately mitigate) Specter and Meltdown which were almost certainly hardware bugs.
I’ve seen cases where a functional block of an SOC got used one generation to the next without even seeing an issue.
And then suddenly, due to changing boundary conditions, a bug got triggered.
I’ve similarly seen cases where HW bugs were known but SW workarounds where calculates to reduce the chances of them manifesting themselves to be low enough. Until they suddenly didn’t.
The state space of HW is incredible, and it’s often not possible to foresee or understand the interactions between of SW and HW.
As if SW applications that stress the system in some way are capable of triggering a HW bug?
And if the issue eventually does get solved with a SW update, then it most definitely was a SW bug, because software update that work around a HW bug aren’t a thing.