You can search in articles, just not with the SMF search.
Yopu need to use the TP search function, to be found under
../index.php?action=tportal;sa=searcharticle
Or you can add a TinyPortal search block to your page...
Create a php block and use the code snippet for the article search..
global $context, $scripturl, $txt;
echo '
<form accept-charset="', $context['character_set'], '" name="TPsearcharticle" action="' . $scripturl . '?action=tportal;sa=searcharticle2" method="post">
<div>
<input type="text" style="font-size: 1.3em; margin-bottom: 0.5em; padding: 3px; width: 90%;" name="tpsearch_what" /><br />
<input type="checkbox" name="tpsearch_title" checked="checked" /> ' , $txt['tp-searchintitle'] , '<br />
<input type="checkbox" name="tpsearch_body" checked="checked" /> ' , $txt['tp-searchinbody'] , '<br />
<input type="hidden" name="sc" value="' , $context['session_id'] , '" /><br />
<input type="submit" value="' , $txt['tp-search'] , '" />
</div>
</form>';