Is this actually true? The way the techspot article was laid out certainly implies that prosecutors knew about all the contradicting evidence, but they offered a plea deal. However, if you read the article carefully it doesn't mention any interaction with the prosecutors or that the contradicting evidence was presented to them.
Yes.
It's not that hard to imagine that the legal system is set up to protect the comfort and careers of the people who have to be in court every day, and not necessarily the 99% of people who are in there for a handful of days - total, across a lifetime - because they had a bad day or two, or were unlucky, or need help that society is as-yet unequipped to give them. It does not surprise me that the letter and spirit of the law have a tough time surviving the practical realities of being administered, day-in-and-day-out, by fallible and self-interested human beings.