voted to do yesterday. Everyone knew that permitting voluntary activities on campus would mean some schools would be able to bring players back earlier than others, while other could bring back more players than others. I've linked the story from Louisville's website, which has more details than Forde's tweet.
Conference level discussions have been happening for a few weeks now. If ND or any other ACC school had a problem with one member school starting campus activities before others, they likely would have raised it with the ACC already to see if they wanted to make a conference mandate, as you suggest. This likely isn't a surprise to any of the ADs or coaches in the conference.
Competitive equity, especially concerning summer workouts, is low on the priority list of ADs right now. Health, safety, and mental well-being are the top priorities. Also, these voluntary workouts for basketball and football are going to be weights and conditioning only, no coach supervised or conducted exercises. The benefit of these summer workouts likely lie more in the ability of players to reconnect with their teammates than anything physical they'd gain from a few weeks of modified weight training.