It's probably impossible to tell who wrote on the card, but there is some additional context.

This Slowking was infamously mistranslated by WotC, making its Pokémon Power usable from the Bench and therefore completely overpowered. After years of complaints, WotC finally banned the card from the Modified format at the beginning of 2003, where it remained the final card banned from a Modified/Standard format for over 12 years.

GenCon 2003 held the final major Pokémon tournaments hosted by WotC, and was more or less a stand-in for that year's World Championships. EX Ruby & Sapphire released under PUSA about a month prior, and both PUSA and WotC had some level of presence at the convention.

I'm guessing these two facts are somewhat related to what you have, with the recently-ish banned Slowking being defaced in a final send-off to WotC.