i also don't know what their project is like, but often text files are used for things like credits, story blurbs, etc, which should be loaded by both the game and shown on a website etc, so often it makes sense to only keep one copy of that around, and editing larger texts inside a gdscript can be pretty messy.
for example i have a
LEGAL.txt file inside my latest project which can (in theory) be read both on the project's gitlab page (which is set to private, so in practice it probably wont) and by my code to display the credits (which is where players will see it). just easier to type stuff like that into a text file (technically markdown, even made a rudimentary markdown-to-bbcode parser to render that to a RichTextLabel :P) than just vomiting it right between your code :P separation of data and code being a thing and all that.
plus unless you encrypt your package, long texts inside
.gdc files can still be perfectly read out using a tool looking for ascii sequences such as
strings. and if you encrypt it it should also encrypt the
.txt file packed inside it so that should be fine.