Bonnie you need a floating selvedge for twill. You can add one on each side even after you’ve beamed. Just measure 2 warp threads( I often double them) and weight them off the back beam. Sley them through the reed but don’t thread through a heddle. They will float on either side of your shed.
It is possible – depends on the threading / treadling combination and thoughtful consideration of which side you are throwing the shuttle. You might have lucked in to the perfect situation without realizing it.
But most of the time, a floating selvedge is needed to capture the outside threads of your warp.