What's confusing to me is that my password was never used elsewhere (it was generated only to be used with LastPass and stored in KeePass). Other reports here say that their passwords were unique as well.

I just have a doubt right now about the possibiliy that this attack was using passwords from past breaches (which is what LastPass is saying)

There are several recent vulnerabilities which could have resulted in your computer being infected with malware without you knowing (like the log4j vulnerability). Because you're storing your passwords in a KeePass vault this actually increases the platform size for attack. This could have taken the form of several fairly simple attacks, such as key logging, clipboard (copy & paste) sniffing and quite a few other methods of stealing your master password purely because you've stored it somewhere other than your brain. Given the number of reported events in recent days, this looks more like individual compromisation events (malware/viruses locally on each affected users computer) than a single large breach.

It's also entirely possible this is all is due to an entirely new vulnerability which hackers have uncovevered which the security community has not recognized yet. This is less likely, but whether it is the case or not doesn't change the fact this likes like a higher than average incident rate for indivual compromises, rather than a larger single event.

But when they are talking about breaches they aren't just referring to other web sites being hacked. In theory, your computer may have been compromised some time during the last years.

It was just weeks ago some very popular package on NPM was found to collect credentials.

Again, not saying that's what happened but theoretically your computer was breached with some malware which collected credentials. I just meant it "makes sense" from a technical point of view. The likelyhood of this being the issue I am more unsure about.

Understood, and that makes sense.