I never worked with the modulo operator. You can easily get what you want and be more flexible with an array. Something like:
HTML:
<xf:if is="in_array($post.position, [2,5,9,15])">
Show content...
</xf:if>
Edit: You should be able to use the modulo operator in that way:
HTML:
<xf:if is="$post.position % X == 0">
Show content...
</xf:if>
->
X
is for "every X post"; e.g. 2 for every second post.