When does Google use article published date vs article modified date in SERP?

Historically, Google said they ignored structured data which was not used to markup visible content.

Because your snippet shows both date published and date updated as visible on-page content but the Moz example shows date published as non-visible meta data, I'd try tweaking that and seeing if it works to get Google to show the last update date instead.

Here's the final mark-up I have on my page:

<div itemscope itemtype="http://schema.org/Article">
      <meta itemprop="datePublished" content="2014-05-09T05:40:51+01:00">
      ... Page goes here...
      <time itemprop="dateModified" datetime="2015-02-22T14:55:06+00:00">Last Updated: 22 Feb 15</time>
</div>

Which gives me a in serp date of

22 Feb 2015