[tlb]

You can't use a straight hash in applications where the plaintext might be guessable. In the example given, you might try hashing "heads" and "tails" to see if that matches the SHA you were given. The random padding on an HMAC replaces the bit about lasagna in the example.

[tuxxy]

The plaintext isn't guessable, though. The post says they use a 32-byte random string.

[maxtaco]

Agreed we could have used either of those. We needed a PRF keyed by the Game ID (so the revealed secrets of this game can't be replayed in the next). Blake2b and SHA3 would also have worked fine.