Duverger’s law is about how there tend to be two parties.
Emphasis on the ‘tends’. It’s a probabilistic observation, not a law of nature. Treating it as the latter leads to people acting against their best interests.
Sri Lanka has ranked ballots. It’s not a Plurality voting system.
You are right, in theory, but please check how many additional votes the winner (or the runner-up) got as second-prefrence votes. It was around 2% of their totals. This is because in practice, most voyers didn’t bother putting second and third preferences.
Or maybe it’s bullshit, designed to scare people into voting for bad candidates.
Even Duverger himself did not consider it some universal law, merely as a statistical trend. A better formulation would be ‘under these conditions, a third party has difficulty forming and attracting voters, and an established party can survive longer than it should, purely based on merit’. Says the exact same thing, but cannot be misinterpreted as easily.