To configure DITA content as an article in Fluid Topics, give it the ft:editorialType metadata key with a value of article.
For example, in a DITA map:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE map PUBLIC "-//OASIS//DTD DITA Map//EN" "map.dtd">
<map title="...">
<title>...</title>
<topicmeta>
<othermeta name="ft:editorialType" content="article"/>
</topicmeta>
<topicref>...</topicref>
</map>
The ft:editorialType metadata key can only be at map level. With a top map gathering several maps, tag the top map only.