Rising sea levels don't mean that people look out their windows and see the water getting closer year by year and then, if they are smart, decide to move. I don't think the comment you're replying to put it very well referring to millions of drowned Bangladeshis but read the rest of the article you linked to. It talks about things like increased flood risks and loss of groundwater and arable land. It would take not very much of those things to have very severe consequences for Bangladesh, in the form of many killed and many more displaced and impoverished.
The "not smart enough" comment borders on the offensive. The severity of floods is often impacted by environmental changes, particularly by loss of habitat that soak up water (trees and wetlands), many point to this as a big impact in the Katrina floods. Are these type of flood victims not smart enough because they didn't project the increased risk from a change in their environment?