The phrase "anhydride is acidic oxide" is correct within a certain range. Anhydride is formed by acid dehydration, while acidic oxide is an oxide that reacts with alkali to form salt and water. Conceptually, the two are not equivalent. In inorganic chemistry, acid anhydrides are formed by acid dehydration, and most of them are oxides, so the two can be equivalent, but in organic chemistry, many organic acids are not oxides after dehydration, such as: acetic acid (CH3COOH) dehydration to form acetic anhydride ((C4H6O3,(CH3CO)2O), so it cannot be acidic oxide.
Anhydrides of common acids:
H2SO4 → SO3
H2SO3 → SO2
H2CO3 →CO2
H3PO4 → P2O5
HNO3 → N2O5
HNO2 → N2O3
CH3COOH → (CH3CO)2O
H3PO3 → P2O3