That is correct. Electrum doesn't follow BIP39 standards, though they use the same wordlist for convenience sake, they are not interchangable though. The entropy is used to determine the mnemonics, which is the 12 words given to you. That is the sole purpose of the wordlist. There is thus no point restricting the passphrase (aka. 13th word) at all, Electrum also doesn't restrict the length (IIRC).