What should I do as a reader if a paper misrepresented earlier papers?
In general, there is not much you can do. If a misrepresentation leads to erroneous results, one way is to write a "letter to the editor" straightening out the error. Such an approach does not seem right in your case since the error is to state that someone has not reached conclusions they really have. I do not mean to say that this is ok but it is harder to get a comment published when the problem, for example, does not affect future research by introducing errors. One also has to bear in mind that the error apparently escaped both reviewers and editors on the way to publication. It may also well be that the author actually see it the way they write it which means the misrepresentation is not intentional (as you seem to imply). So in the way I read the particular case, I cannot see an immediate way forward. If one writes about a problem where the conclusion you refer to is cited, it is of course possible to point out who was first and thereby contribute to setting the record straight.

So, if a misrepresentation leads to errors, it can usually be pointed out in a published letter. If the misrepresentation concerns a non-critical issue, it is still just as bad, but the possibility of publishing a clarification is much smaller. Now, clearly, these issues are on a sliding scale, so it may be difficult to see where a correction may be reasonable. In such cases, the editor of the journal where the paper was published should be able to provide some feedback on how to proceed.