I looked at a 2012 Sonic for my daughter. It also has the hill assist. As it was explained to me (did not test it myself), the hill assist only holds until the accelerator is pressed and then it releases the brake. According to a Sonic brochure, if you turn off StabiliTrak it will disable the hill assist also.
67 RS/SS 350 auto (owned since 81.)
97 V6 5-speed Coupe (work car ordered new)
2012 1LS (V6 6-speed) Coupe (new work car)