Lightning Network already uses millisatoshis. Of course they can't be settled in sub-satoshi amounts on the main chain, until there's enough interest in a fork to do so.
The fixed supply describes the total sum of units that have been issued, and that are intended to be issued in the future - it doesn't relate to the divisibility of those units.
The fixed supply describes the total sum of units that have been issued, and that are intended to be issued in the future - it doesn't relate to the divisibility of those units.