Correct. Well, to be precise, you also need the seed S (besides the master public key and a single, non-master private key). Electrum uses the master public key itself as the seed, while Armory uses what it calls a chaincode. However, this doesn't make a difference because for practical purposes, i.e. in order to be able to generate the chain of public keys, both the master public key and the seed will be stored together on the same machine.
what you write is correct, but for Electrum users, the word "seed" refers to a secret number used to derive their master private key.
what you call "seed" here is Electrum's master public key.
I am just writing this in case users who are familiar with Electrum find it confusing. :-)