I was always under the impression that this was a very well known error card but not very popular compared to others, i picked up a few for next to nothing a few years a go on ebay just for fun as they were usually not very hard to come across albeit not listed as the error. It’s been on Bulbapedia for as long as i can remember in the list of error cards,
I’m not sure why someone would question its authenticity but i’m sure if you sit through a few base set unlimited box opening videos you can find one being pulled from a pack.
Here is a Masters Guide for Pokemon WOTC Corrected Errors & Test cards which has it included as well as many others. It’s full of helpful info! 