To be delusional means to have false belief
It is an undeniable objective fact that modern science is not the actual truth, rather it's a highly inaccurate incomplete image of what the truth is. Science is a very useful tool, but not the actual truth. This is because modern science can only determine what's within empirical testability, which is why there are many unknowns.
If an atheist believes only modern science is the truth, they are therefore entirely delusional. Instead of holding the correct non-deluded view that modern science is a useful tool, and not the truth, they take the delusional belief that modern science really is the truth.
Otherwise, if an atheist did not believe that modern science was the truth, they wouldn't be an atheist, they would be at least agnostic.
Therefore we can correctly conclude that every atheist who believes that modern science really is the truth is nothing more than a delusional.