I do not believe what they say that Satoshi did not know about this, i'm sure that was discussed back then, i certainly knew and i am no cryptographer or cypherpunk tho i did read their writings from time to time.
I'm sure Satoshi was aware that computing power might improve over time and that cryptographic standards might become vulnerable. That possibility just wasn't worth burdening Bitcoin with something so unwieldy as Lamport signatures, which are much worse for scaling. He probably figured, "cross that bridge when we come to it."