I think the idea is that the master password can be memorable enough by picking say 6 random diceware words and still strong enough to resist all (pre-quantum)

I think the idea is that the master password can be memorable enough by picking say 6 random diceware words and still strong enough to resist all (pre-quantum) brute-force attacks for a long time. The arithmetic then looks like this: (7776 ^ 6) = 221073919720733357899776, because there are 7776 words in the diceware list. (7776 ^ 6)/(10^12) = 221073919720 seconds to try all such password if the imagined hardware accelerated hash cracker can do one trillion hashes per second. In years this is: (7776 ^ 6)/(10^12)/(365 * 24 * 60 * 60) = 7010 years. So on average, it will take about 35 centuries to brute force the password when using hardware that can try one trillion guesses per second.