Why can't Mg(OH)2 act as a buffer?

I’m so confused. Buffers consist of a weak acid/base and forms it’s conjugate base/acid in solution. Mg(OH)2 is a weak base, and when a strong acid is added to it, it forms it’s conjugate weak acid and water. Since it’s a conjugate acid and base, wouldn’t that make it a buffer?

I’ve been on this homework question forever and just need an explanation. Mg(OH)2 is an antacid by the way.