> divs are as semantic as you want them to be.

Then they are not semantic at all.

<div class="content">, <div class="post-title">, <div class="meta-data"> all have semantic CSS class names. But they are not semantic HTML names. Remove the CSS, and all you have is <div>, <div>, and <div>. This makes it difficult for screen readers (and web surfers who don't use CSS (are these people even real?)) to understand what the page is saying.

However <article>, <header>, and <aside> all have semantic meaning that can be discerned whether or not CSS is used.

Also: Whether or not something validates is not a good indicator of it being "semantic".