Nah, that's rubbish.
When an ACL snaps it is removed and likely replaced by a strip of tendon (usually hamstring) which has a stronger tensile and shear strength than an ACL. People tend to lose what is called proprioception which is the bodies ability to sense as they did before in the joint (affects balance and the such like). Nowadays with surgery and rehab starting almost immediately then this loss is mitigated to a certain degree.
What really affects ACL (and PCL) injuries are the muscles and damage to the structures around it. Jonny has huge thigh muscles which will see him in good stead for this, particularly the teardrop shaped muscle on the inside of the knee (vastus medialis) It is highly unlikely that anybody tears their ACL without tearing their cartilage (meniscus) on either side of the knee. If he hasn't then the ACL shouldn't affect him long term. If he has torn the cartilage a little on either side then his knee may not be great when turning and it all depends how badly it is torn. If he's torn his MCL then that is a bad one but it seems like he didn't do this and this has not been mentioned.
It is really only bones that grow stronger after damage.