Can a Peer-Reviewed Article Claim Novelty in Something Introduced in a Preceding Phd Thesis?
Of course, they can claim novelty -- and of course, this would be not true. It would also not be true if you published e.g., in a blog. (And of course, if they are aware of the thesis, they are not behaving ethically.)
However, if your PhD thesis is not very visible (most thesis are not), it could be that they are not aware of the thesis and the referee is also not. Most likely, you cannot do much then afterwards to change the claim in the paper -- if you are very lucky, the writer posts some kind of erratum on their homepage.
Many peer-reviewed papers were forgotten and ideas rediscovered afterwards. Of course, this could also happen with a PhD thesis.
Wouldn't it be a strange world if it was okay to take theorems out of a PhD thesis and claim them to be one own's theorems?