Student or Learner
a. He didn't care about it, just as I did.
b. He didn't care about it, as I did.
Can't these sentences mean two things:
1. I did care and he didn't.
2. He didn't care and neither did I.
b. is ambiguous. But it can be disambiguated in speech by stressing did.
With the comma, b tends to mean 2. But it's more likely to be a badly written sentence intended to mean 1.
Without the comma, b means 1.
He didn't [care about it as I did.]