No it isn't. There is no reason whatsoever to suppose Phobos isn't a natural phenomenon.

On the Phobos blog, you can find a recent post (http://webservices.esa.int/blog/post/7/1085), which says:

  [..] provide meaningful new constraints on the
  corresponding range of the body's porosity (30% ± 5%),
  provide a basis for improved interpretation of the 
  internal structure. We conclude that the interior of 
  Phobos likely contains large voids. When applied to 
  various hypotheses bearing on the origin of Phobos, these 
  results are inconsistent with the proposition that Phobos 
  is a captured asteroid.

In other words, the moon is pretty porous and asteroids aren't, so it isn't a captured asteroid. However, that is only one of three hypotheses. In a previous post (http://webservices.esa.int/blog/post/7/989) they state:

  The origin of Phobos is a mystery, in fact three scenarios
  are considered possible.  The first is that the moon is a 
  captured asteroid; the second is that it formed in-situ as 
  Mars formed below it, and the third is that Phobos formed 
  later than Mars, from debris flung into martian orbit when 
  a large meteorite struck the Red Planet. Among other 
  objectives, the Phobos flybys are designed to provide 
  clues towards answering this question.

So one of the three hypotheses has been ruled out. The other two are still candidates. There is no mention of any internal voids with inexplicably 'geometric' shapes