As I understand, this tax scheme is intended for "non-Italian-sourced income".
This explains what "non-Italian-sourced income" is:
An source of income is deemed to have originated abroad (with a “mirror reading” of Article 23 TUIR and without prejudice to that provided by international tax treaties) when
- OR the asset generating the income is situated abroad,
- OR the business generating the income was conducted abroad,
- OR when the individual remitting the income is resident abroad for fiscal purposes.
Only one condition has to be fullfilled.
There is no mentioning of investor. They speak about ANY KIND OF INCOME.
In any kind of trading/investing, the assets (brokerage accounts) are outside of Italy, so one condition is fulfilled as indicated in the fiscal law.
The main reason why they speak about the "non-Italian-sourced income" is to avoid that Italians would try to use fraudulent setups to abuse this flat tax rate for existing businesses in Italy. They could then reduce the paid taxes massively.