Not necessarily true. You can get private key in Electrum wallet too. Seed key is for back up the whole wallet while you can get private key of any of your addresses from Electrum too, not only in Bitcoin Core. Each address can be accessed through their private key. To get private key in Electrum, select and click on right button if your mouse and get private key.
In bitcoin core, as wallet back up, you have to save the wallet.dat file. While in electrum, to save the wallet, you can save both wallet file and the seed key.
On the other hand, in both bitcoin core and electrum, you can get private key for each of your addresses.