To say that one "can't help" doing something means that she can't not do it – that is, she has to do it or must do it. Here, that expression is used in a rhetorical question. "How could one help agreeing?" means "how could one disagree?," with the implication that one couldn't possibly disagree (because the logic underlying Stolkin's assertion is irrefutable).
You should note that this is a special, limited, and somewhat counterintuitive usage of the verb "to help," in which "help + [verb]ing" means "avoid [verb]ing" or "refrain from [verb]ing." In this particular usage, "help" is almost always used in the negative potential form, unless it's occurring in a question (as in your example). That is, we say "I can't help [doing something]" or "I couldn't help [doing something]"; we would almost never express the idea "I can avoid [doing something]" by saying "I can help [doing something]."
Also, sometimes instead of "can't/couldn't help [verb]ing" you will see "can't/couldn't help but [verb]."
Some simple examples:
You say you've told me everything, but I can't help thinking you're hiding something from me.
She tried to look serious, but the expression on his face was so funny that she [couldn't help laughing / couldn't help but laugh].
When you heard what a difficult life he has had, how could you [help feeling sorry / help but feel sorry] for him?