Yes you can actually use just any random words you like to create a seed phrase but most of those wallets that uses BIP39 standard will not be able to recover that wallet, as for the checksum it can easily be solved and then pick the word that matches it. But it is bad idea to actually manually generate your own seed phrase because it doesn’t create the randomness that your wallet or Machine will pick
No, that is not what that post meant. Sufficient entropy is required to generate the mnemonic using BIP39. The acceptable size of the entropy is between 128bits to 256bits, and hence you can actually just use the SHA256 hash function of hello as the input for the entropy, generating a rather insecure mnemonic.
To calculate the addresses (as well as the corresponding private key), we use a KDF on the mnemonic to come up with the seed. For example, using SHA256 of "hello" as the entropy yields:
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