His example is unclear, but he's complaining about the behavior of conditionally casting the initial data values based on pattern matching.

   <div id="test" data-foo="1.1"> 
   $('#test').data('bar', '1.1');
   typeof $('#test').data('foo'); // number
   typeof $('#test').data('bar'); // string

If the data-foo value is user input, then converting it to a number might not be expected. E.g.

   <div data-emoticon="0.0" />