I understand it to be geopolitically significant. Biden, head of state for a big country. Xi, head of state for another big country. China's made it known they don't want Xi called a dictator. Biden's done it anyway.
It's the scale of the thing that matters here. It's a political statement of gravity. He's calling any bluff they've got going, and demonstrating that the USA doesn't respect China enough to use their preferred nomenclature. It's a calculated insult and a challenge.