I don't think that's exactly right. It's true that neither Factionless nor Divergents fit into the system, but trying to compare the two states is sort of like comparing apples to oranges. Read on:
Recall that the aptitude test does not determine what faction you MUST become part of - only the faction you are best suited to join. At the end of the testing process, you can still choose to join any faction you like (including remaining with your parents in the faction you were born to.) Although, common wisdom in this society would suggest that choosing a faction that's different from your test result is likely to leave you Factionless.
With that in mind, being Factionless does not necessarily mean the person didn't belong anywhere. They may well have belonged in one faction, but chose to join another and subsequently fell through the cracks. OR they could have joined the very faction their test suggested and STILL failed the initiation process (this is most common in Dauntless.)
There are also those who CHOOSE to become Factionless for personal reasons. In fact, some people might choose to become Factionless precisely because they ARE Divergent, either because they cannot cope with living in just one faction, or in an effort to avoid persecution should they be found out.
Being Divergent, however, is something different. It's not really something you can choose to be, nor is it a status that society can foist on you. It's a side-effect of your brain working in a way that makes you incompatible with the aptitude test - such that you don't get a clear reading. It's not a superpower, it's just an anomaly in the testing process. One that makes you an abomination in this society, because of the value they place on this particular test.
Being Divergent means that you are "normal" by our=world standards. Remember that the experiments are made of people that are "genetically damaged," so since a Divergent is a healed person, then a Divergent is technically just a genetically normal person in our world.
A Divergent person could also fail the initiation of their chosen faction and wind up Factionless as well, just as easily as a "normal" person. Tris didn't really do herself any favours by choosing Dauntless, in that regard. Luckily for her, she had pluck when it counted and passed anyway.