It seems the formula might calculate the input shape as:
input_size = [(3, 160, 160), (3, 160, 160)]
batch_size = 1
total_input_size = abs(np.prod(sum(input_size, ())) * batch_size * 4. / (1024 ** 2.))
print(total_input_size)
> 6116.0 # 6GB

which is wrong as the memory from multiple inputs should be summed not multiplicated:
print(sum(input_size, ()))
> (3, 160, 160, 3, 160, 160)

I would recommend to create an issue in the repository and explain the concern there.
PS: I’m not a big fan of tagging certain people, as this might discourage others to post an answer.