I am going to have to disagree with this statement. For the deadlift, active knee flexion is useless, but in all around knee function and athletic activity, knee flexion is very important. When it comes to ACL repaired knees, hamstring strength, especially distally at the insertion on the tibia, is very important. Among other things (including gluteal strength to avoid a valgus knee position, proper ankle and hip mobility, etc), distal hamstring strength can help prevent and resist anterior tibial translation, which is what the ACL's purpose is as well. Decreased/insufficient strength of the distal hamstring = increased load on the ACL when an anterior translation of the tibia occurs during activity.
Now I am not advocating performing huge amounts of isolated knee flexion/hamstring curls as prevention work for ACL injuries, but you can't ignore that function of the knee as well. With proper exercises like glute ham raises, sprinting (when appropriate for ACL repaired knees), swiss ball curls, slide board curls, etc, you can incorporate hip extension with knee flexion strength and develop proper motor patterns at both ends of the hamstring. If you develop strength at the hamstring into hip extension, it will help develop all around strength of the hamstring which may help with knee flexion, but you can't just ignore the motion. Obviously, the glutes and hamstrings work synergistically for hip extension and proper glute function is a huge priority for healthy knees. But if there is a weakness associated with knee flexion, you can't ignore it.
Also, for all around knee health and injury prevention, you don't want weak hamstrings in the knee flexion action. Weak hamstrings in the knee flexion action can result in the IT Band taking up the slack in that motion and becoming over dominant. That can lead to other issues, including IT Band friction syndrome and other pathologies up and down the chain due to stresses placed on the lateral knee and fibular head and tightness in the hip due to a tight IT band leading to a tight TFL.