I agree that all the signs point to it. However, in the movie everyone appears convinced that an Obscurial would not be over the age of 10 and it is indicated that the older Credence must be incredibly powerful to have survived so long.

Had Ariana been an Obscurial, **Grindelwald should have been less certain of the age of his target**. Instead, he explicitly tells Credence to look for a younger child and appears genuinely surprised when he reveals his power.

Furthermore, it appears from the movie that killing an older Obscurus requires a concentrated and sustained effort by multiple adult wizards. This is inconsistent with Ariana's quick and accidental death.

Still an interesting idea!