Try

    \newcommand\iddots{\mathinner{
      \kern1mu\raise1pt{.}
      \kern2mu\raise4pt{.}
      \kern2mu\raise7pt{\Rule{0pt}{7pt}{0pt}.}
      \kern1mu}
    }}

This is based off the actual definition of `\ddots` in $\rm\TeX$, which constricts the character from three separate dots like this.

Then the result for `\ddots` is
$\newcommand\iddots{\mathinner{\kern1mu\raise1pt{.}\kern2mu\raise4pt{.}\kern2mu\raise7pt{\Rule{0pt}{7pt}{0pt}.}\kern1mu}}\iddots$ rather than $\unicode{x22F0}$.  Compare to `\ddots`: $\ddots$