In fact, the uptime of US nuclear power plants was above 90% for the last decade.

And even if a reactor goes offline, a power plant usually operates 2 to 4 reactors, so the entire plant continues operating.

This is a brand new type, there's no way that equivalent operation to a decades old design with centuries of operational experience can be assumed. Presumably its been designed for high uptime but it would not be unusual for new technology like this to require some refinement.

Is 90% equal to 24x7, 365?

Unless they hire Homer Simpson.....