So, according to me, the Purple Yoshi tech demo is actually fake media created by some people for show. It was not a tech demo at all. It was just concept art showing the Game Boy Advance's capabilities through a non existent video game.
The concept art shows a purple Yoshi with a much longer body (only his back is seen) with 4 arrows pointing at one of the spiders. Based on this, I think the tech demo was created as a shooting game reworked into an unknown non-Yoshi game.
I almost solved this mystery a couple years ago when I looked into people who contributed to IGN's site. The story of the image is that it was some kind of exclusive Japanese content that no one else had managed to get, but IGN's Japanese correspondent did and shared it on the website. However, it was blurred out in the article mentioning it and could only be seen on the forums.
Their forums had just launched at the time and considering the strangeness of the image, it's believed to have been completely faked by IGN simply as a way to get people to go to the forums. However this was never proven but finding out who that correspondent was could solve the mystery.
A couple years ago, I did a search for old IGN writers that had resumes and job descriptions that listed what they used to contribute, and I actually did find a guy who was their Japanese/foreign correspondent. I sent them an email and unfortunately they never replied but the email could have been dead and I'm not sure if I tried anywhere else at the time.
It's something I could look into again and would probably get us an answer. And if IGN didn't make the image, I would really like to know where it came from or who made it. As far as I know, there hasn't been any word on it's actual origin other than the likelihood that IGN made it.