Proactive secret sharing security flaws
Reading the quoted section and then looking at the notes, it seems that this is not well explained in the notes. However, my assumption is that the adversary has compromised s2 before PSS, but then lost access and therefore does not have access to s2'. If the adversary had access to s2', they wouldn't need to do the attack to get s2'.

So, the adversary has s1 and s2 from before PSS and gets s3' (and s1') after PSS. In neither case does the adversary have the required 3 shares. But, because the adversary knows h(x), it is able to compute s2' from s2. Which allows the adversary to recover s.