That's correct, although with some variants you specify charset and length so there end up being tens of possibilities (lcase? ucase? digits? symbols? length=8,

That's correct, although with some variants you specify charset and length so there end up being tens of possibilities (lcase? ucase? digits? symbols? length=8,10,12,16...) rather than just one. The idea is that you use a passphrase that's reasonably secure, so that it cannot be brute forced. I used to use a similar password hasher browser extension, and I think it's fine as long as the seed passphrase is sufficiently secure. However, now I prefer keepass/lastpass/1password with random passwords. Not having to type in the passphase as often makes it easier for the average person to use a more secure passphrase.