I've heard this "fact" presented many times, but is there actually any meat to it? It seems to me there must be many mature and well-debugged phone stacks out there.
Now Nokia has put out hundreds of models of S60 phones, each largely sharing components. I would imagine that their telephony layer was rock solid. I have no idea if the assertion in the article is true or not, but it wouldn't surprise me. Nokia has always "gotten" the phone side... it's the UX side of a smartphone that has always vexed them.
True, but my current phone N900 is weaker in terms of a phone. It is a hacker's delight with a real terminal, apt-get etc but the phone experience I never really enjoyed much. My earlier phone N95 was better than this.
What makes Android successful, was coming late to the party are QCOM & co having easier to integrate solutions with their baseband chipsets.
In the past, Nokia was one of the first to have a prototype implementation -- hence, more time for testing and improving it.
I would say this is no longer the case today -- but it's still probably one of the more mature implementations around.