I'm fairly sure these restrictions do no apply to Akrafjallsvegur itself. Looking at the signs near the intersection of Akrafjallsvegur and Akranesvegur (https://ja.is/kort/?x=353234&y=428738&nz=13.52&ja360=1&jh=116.7) they all have a sign below stating an exception to the upper sign e.g. "No horses (upper sign) - í Hvalfjarðargöngum (lower sign)", meaning that the restriction only applies in the Hvalfjörður tunnel.