I don't think she's claiming that Shakespeare isn't a credible source, just that his plays provide only one take on events (also, they punch it up a lot in order to make it play better on stage. I'm sure the real Macbeth didn't run around making deals with witches). If someone based their whole view of 15th century British politics on his work, it would be like making assumtions about the Napoleonic war based solely on Bernard Cornwell's Sharpe series.
As far as Richard III goes, the fact that Elizabeth I, the ruler of England when Shakespeare was writing Richard III, was descended from Henry Tudor, whose claim to the throne was shaky at best and who basically got to be king by having Richard III deposed, probably played a part in Shakespeare's decision to depict Richard III in a negative fashion. If you're writing a play about the guy the current bunch of rulers kicked off the throne, it probably pays to make him look like the bad guy, and show the people currently in power in the best possible light.
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As far as Richard III goes, the fact that Elizabeth I, the ruler of England when Shakespeare was writing Richard III, was descended from Henry Tudor, whose claim to the throne was shaky at best and who basically got to be king by having Richard III deposed, probably played a part in Shakespeare's decision to depict Richard III in a negative fashion. If you're writing a play about the guy the current bunch of rulers kicked off the throne, it probably pays to make him look like the bad guy, and show the people currently in power in the best possible light.