That's a broad claim, probably not supported by the actual blog post, but what the heck. I was paging through a 1955 New Yorker - the date is important - and came across this.

If you're over 50, you probably have one reaction: oh, it's a government worker on vacation. If you're under 50, you might think "was that by the same guy who drew Rich Uncle Skeleton?" It's possible to have both reactions. 

I'm 100% sure the artist is Howard Wilcox, who worked for Cunningham and Walsh; the history of his later creation says he drew it first for Chase Manhattan Bank. Chase and clumsily-named the Bank of the Manhattan Company merged in '55, and it appears the ad agency kept the account.

Almost nothing on Mr. Wilcox on the internet, alas. Seems he deserves a bit more, given that he created one of the most ubiquitous characters of the 60s and 70s.