Sphinx of the Second Sun's extra Beginning Phase removes Summoning Sickness

If [[Sphinx of the Second Sun]] is in play and its controller summons a creature (without haste), that creature won't be summoning sick anymore as soon as the extra beginning phase starts. This is in contradiction to rule 302.6 because the creature has not been under its controller's control continuously since their most recent turn began, even if a beginning of turn phase happened.