Is the 2nd line really necessary? Since β is 0.9, the second line becomes:

c ← 0.9m + (0.1)g

There doesn't seem to be a significant difference between their code:

θ ← θ + η sign(0.9m + (0.1)g)

and just not having the 2nd line at all:

θ ← θ + η sign(m)