Of course it might be that OP evaluated that and decided against it for good reasons.
Ethernet just works it's an established protocol and there aren't that many easy ways to transfer files other than it today that work out of the box and with any combination of hardware and software.
Serial I/O is common Hello World example for Raspberry Pi. I think you have to worry about less things than adhoc wifi networks.
> likely much slower
It's slower, but not much slower. The baud rate can be set up to 4,000,000, where a megabyte would transfer in 2 seconds. These "gifs" don't seem to be more than 10 seconds long, so they should be less than 1 MB, judging from random Giphy mp4s.
No, connect them via the USB and configure one of them to appear as a USB Network Interface. Point-to-point connection established. Plenty of HOWTO's available.
My point is that making those types of custom, physical connections are kind of a manufacturing nightmare. If the hobbyist that did this was able to do it with WiFi with no need for more manufacturing and making sure the sled would be able to reliably push the pins in every time, then WiFi was the right call in my eyes!
I wonder if there's not an off-the-shelf connector that can be used to route power and USB signals which would fill the role. There's nothing saying the connector has to be small, the entire back and underside of the cartridge is available for the purpose. The original Polaroid film packs had two large pads on the bottom and the camera two long springs to connect with them to provide power from the pack to the camera. However, Polaroid didn't have to contend with ejecting the entire pack with every photo.
My bias says that using wifi adds complexity, but I can see that getting the physical connector right is also not as simple as it first appears.
Edit: Oh dear, I see you're the creator. I guess I misunderstood. How do the pins play in to the design if not to connect the two devices?
Also, you replied to the creator of the camera, so I'm going to guess he knows best :)