Well, "legality != morality" basically just boils down to leaving us with no instructions as to what morality is. It can be an argument for anything. Slavery is illegal now, and legality isn't morality, so what?

Unfortunately, all these things exist:

• legal and moral things
• illegal and immoral things
• legal and immoral things
• illegal and moral things

So they really give us no information about each other.

I don't think the post is insinuating that something being illegal constitutes it being moral either... I think it's just correctly pointing out that there is no meaningful connection between legality and morality.