My friend got back to me and said that the charizard was from staff decks and given away at the event they were used at. My friend thinks it’s probably a tournament, but he didn’t see anything about what event it was. That’s most likely where the other unlimited cards come from as well.
On that blog you linked is another post about unlimited cards showing a couple others cards:
choco7maron.blog.fc2.com/blog-entry-148.html
I think this post explains how they were for teaching kids how to use the e-reader cards at pokemon centers/tournaments and after teaching the kid they’d be given the card? I could be wrong though since my google translate-fu isn’t the best. I could try asking my friend to translate this post as well but I don’t want to bother him too much since he’s not into pokemon cards lol