Wouldn't not believing in the Hajj be equivalent to blasphemy, which in turn is equivalent to being killed in the name of Islam?

Not if all the Muslims around you also don't believe in the Hajj. Orthodox Muslim belief is highly rigid today, but the medieval era there was a time and region when Muslim concepts could be questioned and given some mystical revision instead. That is where Nizari Ismailism (and the Druze who don't even regard themselves as Muslim any more) came out of.