The sea level rise (SLR) maps presented in En-ROADS are risk maps of potential flooding. They do not show land area that will absolutely be under water in a particular year.
The dark blue “Land at risk” area represents the land that is below Mean Higher High Water. This means that the land is below a typical high tide elevation in a particular year and would experience daily flooding. If the area is protected by levees or other flood protection measures, then it would not experience flooding in a typical year. If those protections were absent or were to fail, then flooding would likely occur.
More information at Climate Central.