More related to the article, you would think that subtitles are literally the easiest file format in existence to safely handle. It's incredibly well-defined in terms of textual data and times.
Well, which one of them. There's nearly a hundred different subtitle formats, and each one has a whole set of variants. Just Timed Text alone (XML) can have more layouts than one could count, specially since it's meant to be able to replicate technically all previous industry formats.
Even the DVD subtitle format, which is just a mostly transparent image overlaid on the picture? In XML?
On the other hand, images and videos are likely to be handled using some library, which might be better at safely handling the files.
http://trac.opensubtitles.org/projects/opensubtitles/wiki/Ha...
Depends on the format. SSA for instance can have embedded font and image files, which presumably have much more complex decoders.