Many countries with FPTP still have large 3rd parties

The "law" says that 2 main parties tend to emerge. In Canada only once has the prime minister ever come from a party other than the Liberals or the Conservatives. That was in 1917 when the main issue was conscription, and the pro-conscription "Union" party won over the anti-conscription Liberals. It's pretty clear that in Canadian politics there are 2 main parties, and a few other parties that cling to survival.

Occasionally one of the parties ends up imploding, but Duverger's Law is so strong that normally it's only a short time before the duopoly is re-established. In Canada, Brian Mulroney and his party were so unpopular that it caused the Reform Party to form from disaffected conservatives. That meant that in the 1993 election the "Progressive Conservative" party managed only 2 seats in the federal election. But, 10 years later, the rift was healed and once again the Conservative party was the main opposition. Then the Liberals self-destructed and very briefly the NDP was the official opposition, but a few years later Justin Trudeau took the Liberals to a huge victory.

Sure, it's better to have a third party with a few seats than it is to have no third party at all. But, I'd hardly say that events in Canada disprove Duverger's law. In fact, they tend to support it. In more than 150 years, despite everything, the two main parties are essentially the two main parties from 150 years ago.