But why do they say that the xpub file is important to recreate the wallet?
What does it mean?
i never said such thing! i don't even know what "xpub file" means. there is xpub which is the extended public key but it is not in file form even in Electrum.
i said the "keys" are important not the "file". for example this:
5HueCGU8rMjxEXxiPuD5BDku4MkFqeZyd4dZ1jvhTVqvbTLvyTJ
is a private key. you can import it in ANY wallet that has import feature (
this is an example key, don't import this in your wallet!) and they all understand it no matter what version they are, what kind of mnemonic scheme they use, what HD derivation path they use,...
if some day Electrum stopped working you can either export these individual keys form it or ask a trusted python developer to write a script that extracts it from your wallet file or converts your mnemonic (that you have backed up) to keys that you could import in any wallet.