In a way, it's good that the mnemonics aren't stored particularly if they aren't encrypted since the wallet password would then be useless.
But it does show the master private key. There wouldn't be any difference if it had the mnemonic too, despite the fact that you could derive more master keys.
My guess is that since the devs aren't thrilled about BIP39 importing support, judging by the depreciation notice on that window, they glossed over some details that were implemented for Electrum mnemonics, storing them in the wallet file (hopefully with encryption!) being one of them.
They surely aren't thrilled with BIP39, I got my answer here:
BIP39 seed words/mnemonic not stored in wallet file. It has also been discussed 3 years ago:
Seed option is greyed out. Still, I don't find it reasonable to not store the seed. I recently lost a piece of paper containing the seed phrase and thought that I could rewrite it from the wallet file. Warning you that it doesn't generate BIP39 seeds and that it isn't recommended based on their safety standard doesn't mean that they should skip such inclusion.
Question: Besides the 8-bits checksum, is there any other difference with electrum seed generation?