That's not what that clause is about.
Is there some way that I can GPL the output people get from use of my program? For example, if my program is used to develop hardware designs, can I require that these designs must be free? (#GPLOutput)
In general this is legally impossible; copyright law does not give you
any say in the use of the output people make from their data using
your program. If the user uses your program to enter or convert his
own data, the copyright on the output belongs to him, not you. More
generally, when a program translates its input into some other form,
the copyright status of the output inherits that of the input it was
So the only way you have a say in the use of the output is if substantial parts of the output are copied (more or less) from text in
your program. For instance, part of the output of Bison (see above)
would be covered by the GNU GPL, if we had not made an exception in
this specific case.
As the authors state, copyright does not provide a means to restrict the output of a program to the licence terms that apply to the program itself. That is only possible if the output consists of part of the source code of the GPL'd program. (For instance, YACC or Bison generate source code for lexers and parsers, and their output contains large parts of the source code of YACC and Bison themselves.)
But that is not the case for you. The source code of a KDEnlive is GPL'd, but the output you produce with is is not derived from its source code, it's derive from the subtitles you feed into it. Therefore this clause doesn't apply to you.