But did they actually mean literally to use ][ as square brackets. Or is it merely an approximation that is meant to be read as Roman numeral two. And then people in that time writing it as ][ because Unicode didn't even exist yet. And then people stuck with that.

I don't know what the confusion could be. That's the name of the product. It's '][' because that's a creative way to write 'II' which is a fun way of writing '2.' Just like the Apple //c

In my case I stick with ][ for the sake of nostalgia.

Yes, they did. It's written that way in early marketing material and documentation.