memes

You're thinking of topological closure. We're talking about algebraic closure; however, complex numbers are often described as the algebraic closure of the reals, not the irrationals. Also, the imaginary numbers (complex numbers with a real part of zero) are in no meaningful way isomorphic to the real numbers. Perhaps you could say their addition groups are isomorphic or that they are isomorphic as topological spaces, but that's about it. There isn't an isomorphism that preserves the whole structure of the reals - the imaginary numbers aren't even closed under multiplication, for example.