As far as I know, the demonstrative az "that" does not change depending on what comes next, unlike the definite article a(z).

(Exception: if a postposition follows.)

This sentence is something like "200 + 500, that's 700", rather than "200 + 500 is the 700".

Here, "az" is not a definite article but, rather, it stands for "that (is)". Which is a demonstrative, if I am not mistaken.