“ Whoever he be who will rebel against your commandment, and will not listen to your words in all that you command him, he shall be put to death: only be strong, and of a good courage.”
Their covenant was solemn for the penalty for breaking it was death. They agreed that disobedience to Joshua, whether by one or by many, would be punished by death. In a war situation such disobedience would be treason. It could jeopardise the whole venture.
“Only be strong, and of a good courage.” Joshua's part was to have the strength and courage of a good leader resulting from his devotion to YHWH, for that was what YHWH had commanded him. Let him fulfil his commitment to YHWH, then they would fulfil their commitment to him.