Indeed sub-titles and text are different. What David probably means here is:
Make a title and format this title such that is resembles a sub-title (or visa versa). Also make sure that it is precisely in the position of a sub-title.
And then do a fade out for this title.
You will get the desired effect.
Another option is to set the sub-title as required. Do not fade the videopart yet. Separately produce the selection of your video. Then include the produced part into the timeline to replace to original part. The sub-title now is part of the produced video and can as such be faded out.
The downside of these approaches is that the title, cannot be exported as part of the sub-titles, nor made part of a (multi-lingual) sub-title stream on a DVD etc.