If my memory is working, there was a long dry spell of Chinese M14 supply up to 2007, I think it was 3-4 years since a batch had come in. In the interim the practice of putting the dates on the gun had appeared, so as much as they had been arriving in bursts from the late 80's up to the early 2000s, there was no easy way to know which batch yours belonged to. Some of those batches were quite good, some had distinct problems, but you had to look somewhere other than a date code for a clue as to the quality of any particular one.
Then comes the 2007 batch, and they were as good a batch as can be expected from China, and had the date code to clearly mark them. Then the 2009 batch, which was probably every bit as good, except this is when the shorties started appearing in number and those seem destined to have a few issues (normal for modifying a system away from its tried and tested configuration). After that there is a rumour quality goes down at a steady pace, until you get to the bolt breakage stories of today.
So the impression of the internet is that the older your date code, the better gun you have, and 2007 is the oldest date code, thus the reputation. Realistically a 2001-2003 gun is probably as good or better, but much harder to identify.