In BIP39 our "password" is the mnemonic which is encoded from an already strong entropy of between 128 to 256 bits. Adding more "security strength" on top of it is not needed.
I agree with this and it seems logical, but I want to point out that I don't possess enough technical know-how to actually know if your interpretation is correct.
BIP39 seeds are a much more user-friendly representation of a long string that can easily be read by humans. Usually, when you add user-friendliness and simplicity to something, you also compromise security to some degree. How much and if any security at all was compromised with the introduction of the BIP39 standard is something for the more technically-gifted to comment on.