there is one implementation of the function that takes the dice rolls as an input, and uses the SHA256 of that as a seed. SeedSigner.
That's a weird code! It basically generates the random bits but uses it as a string instead of binary! The computed hash is the hash of that string not exactly the same as using the bits directly. The developed possibly had no idea how to convert bits to bytes and compute that hash 
P.S. Using HMACSHA would be useful to combine entropies. For example if you don't want to rely fully on computer generated random or fully on user (dice) generated random you could generate both and then compute HMACSHA256 of them using one as the key and the other as the message.