When it comes to forcing platforms outside of Greece to comply with this, those platforms will just close their service down to Greece.
If you want to talk about the concept itself of removing anonymity: on HN the impact would not be huge, a lot of us are not really anonymous with links to personal sites in our profiles. Reddit is a different beast entirely.
You don't need a physical presence to be subject to another country's laws. Disobeying a judicial order would be grounds for issuing a warrant which could easily be expanded to an international warrant for the owners of the platform.
The "judicial order" in the first place violates the first amendment, which isn't binding on Greece, but is binding on the nature of any extradition order they wish to seek in the USA.
I do wonder how easy it is to de-anonymise a typical reddit user based on topics, ways of writing, time of writing etc. throw into some form of pattern recognition and see if it links up with other reddit accounts, accounts on forums, things like Facebook etc.
Throw in information Reddit has (ip addresses, user agents etc) and it’s no doubt a certainty.
If you want to talk about the concept itself of removing anonymity: on HN the impact would not be huge, a lot of us are not really anonymous with links to personal sites in our profiles. Reddit is a different beast entirely.