There would be a period of internal discussion after author-reviewer discussion period. So my guess would be (if there is no author-reviewer engangement beyond just the first rebuttal), AC (if they are willing to do their due dilligence) will simply push the reviewers privately and ask what their take is on the rebuttal. If in that private discussion nothing really happens (for example, all reviewers just go MIA), then really it might be upto meta-reviewer's personal judgment how they are going to take into account the strength of rebuttals.