It's also about not saying hi
Proper dog to dog socialization should not only consist of dog to dog play.
Yes, it is important to teach a dog how to play with another dog, but that should not be the only thing a dog thinks of when he sees another dog.
We also need to teach a dog how to just hang out and chill with other dogs.
These are very important because in the real world, we cannot have our dogs get into super excited playful mode every time they see another dog.
There are many real world situations whereas they need to just hang out politely and also be able to ignore other dogs when we need them to. (e.g. pass a service dog, in the vet office waiting room...etc)
To be able to do that requires a more disciplined, all-rounded, and matured mindset than a single dimensional play-play-play attitude.
To nurture a balanced attitude towards other dogs we need to teach them how to play, how to hang out, and how to just chill and do nothing - and spend time practice these skills regularly.
This is why it is so important to spend time with your dogs not only on playing with dogs - but also on practicing impulse control through hanging out around other dogs without getting aroused and doing long duration work while ignoring dogs that are very closed by.