> This has been fixed in 7.0.0

It has not been fixed, but rather an improvement has been made to
the function, and improvements are usually not backported.

> as the function is defined, it does not allow my code to
> override it anymore, as in php <= 5.4

In my opinion, it is always problematic to use polyfills. Consider
to use wrappers instead. Something like:

  <?php
  function my_array_column(…) {
      if (function_exists('array_column')
              && version_compare(phpversion(), '7.0.0', '>')) {
          return array_colum(…);
      } else {
          // fallback code
      }
  }

For efficiency, the conditional could be put outside of the
function definition.