The article does touch on HSMs but might be missing the point of them?
> A compromised server no longer means a compromised key
Proper use of an HSM means that even the owner of the private key is not allowed to access it. You sign your messages within the secure context of the HSM. The key never leaves. It cannot become compromised if the system is configured correctly.
You're correct there that proper use means even the owner can't access it. But in a single key scenario they can still act unilaterally. The advantage of TSS is the removal of that level of unilateral action.
Can you point me to an example of a FIPS level 3+ certified device having its private keys compromised due to a defeat of the tamper resistant boundary?
Here are a couple examples of physical access leading to key extraction. You're welcome to be pedantic (those are side channel attacks, they don't defeat the boundary!) but one way or another, physical access wins.
> A compromised server no longer means a compromised key
Proper use of an HSM means that even the owner of the private key is not allowed to access it. You sign your messages within the secure context of the HSM. The key never leaves. It cannot become compromised if the system is configured correctly.