From the article they did say the modifications were "malicious" which implies that they felt they were deliberate. I didn't actually notice that there was reference to a suspected Chinese Govt. tie-in.
I think (I do not know) they decided to not announce the vulnerabilities because they could be exploited by other parties. This is common with software vulnerabilities where they are not announced but are rather provided to the software company to patch before becoming a problem. This wouldn't generally be as simple with a hardware hole. Besides...they could also be reasonably sure that if the hole was only known by themselves and the entity that created it...any future attack could only come from limited sources which could be valuable intelligence.
Also, if they announced it and someone did utilize it for nefarious means then they would be potentially liable for damages. Not to mention that Lenovo would not be happy about the accusations - I have no idea what recourse they might have though. We may see soon.
Finally...do I "believe that leaving people's computers vulnerable to mass Chinese attack is better than warning people; allowing them to take countermeasures and having some inevitable exploits by individual hackers?" I would say no. Nowhere in my comments did I intend to imply that I was in agreement with the decision to keep it quiet. It's kind of like the NSA...I understand why they record everything...even though I'm not okay with it.