I get it... However, bitcoin already has key stretching.
Bitcoin the protocol does not use key stretching.
Bitcoin-core "the client" uses key stretching to harden to the WALLET DECRYPTION PASSPHRASE against brute force attack. Nothing more. Electrum doesn't copy that code from bitcoin-core wallet so it isn't key stretching "again".
edit: actually i dont think it would increase the entropy, just add more stretching, since the seed has less entropy than a 160 bit "normal" priv key
Private keys only have 128 bit key strength. Not 160 bit and not 256 bit.