You're right, and that's a fair catch - I conflated two things that aren't the same problem. Once a file reaches a person, they can leak it; encryption of the transport doesn't fix the human at the other end. An encrypted zip sent over almost anything covers the "moving the file safely" part, no argument.
So let me be honest about what actually drove me, because you've exposed that my post muddled it: it wasn't really fear of a specific design leaking. The design files were just the spark that sent me down the rabbit hole. What actually kept me going was realising how much "free" messaging is paid for with metadata and data harvesting, and not wanting a phone number tied to everything. So the real concern is ongoing privacy of communication, not protecting one file - and you're right that those are different problems. The file thing was the door I walked through, not the room.
Honestly a cleaner way to put it: I didn't build it because zip+email couldn't move a file. I built it because I went looking at how messaging works and didn't like what I found, and then got obsessed. Thanks for making me say that more precisely.