Thanks my seed phrase is safely kept
I believe the Bitcoin wallet you're talking about is not bitcoin core because it has never used seed phrases and the only way to back it up is through the use of the wallet.dat file but if the wallet is like Electrum seed phrase is enough.
When you said the seed phrase is kept safely, I hope you don't keep it in an online device.
Definitely not Bitcoin Core because of the obvious reason you have stated, Bitcoin Core is HD wallet but has no seed phrase that can someone can backup than the wallet file (wallet.dat). Maybe RockBell is just using a seed phrase wallet that starts with the name bitcoin and calling it Bitcoin Core.
@RockBell, if you have your seed phrase, you have your coins, that is just how it works because you can always use your seed phrase to recover your wallet and your coin at any time and as many times as possible as it will recover you the keys needed to access your coins. If your seed phrase is lost, that is all, your coins are lost.
Just backup your seed phrase on a paper, or steel sheet which is more safer to environmental damage.