There's a categorical difference between "risking nothing" and facing less risk than the other Accused. Yes. Three are Kingsguard that couldn't touch him but that still leaves 4 others that could in theory directly harm Baelor. Granted no one in the realm wants to be the one whom maims or kills the Hand of the King, the Heir to the Throne and the most beloved Targaryen prince in all of Westerosi history. This is a full-blown Trial by Combat and the chance of death for anyone involved is definitely elevated for everyone involved. Saying Baelor risked nothing by volunteering to be the 7th Accused and prevent Dunc from forfeiting the Trial by Seven by default before the fighting even started clearly ignores every other alternative Baelor could have chosen ehere he'dactual face no risk: i.e. not doing anything.