People are constantly fighting over this in Gamefaqs...
NINTENDO NEEDS TO CONFIRM IT so noobs can be quiet.
There's an easy way to do this.
Release date of Pokémon Red/Green Versions: February 27, 1996
Publication date of the first volume of Pokémon Special: August 8, 1997
These can be confirmed by reliable sources, namely in this case,
IGN (under the tab in the middle of the page marked "Editions") and
Shogakukan's page for the first volume (i.e., the website of the original publisher of the manga), if you don't trust either Wikipedia or Bulbapedia.
In other words, technically, they have. You'd just need to do some digging to find it.
But to get back on topic, my guess is that with a lot of manga franchises, the manga comes first, followed by an anime and then a game. At least, this is a bit of a leap of logic in any case, considering American volumes of the manga usually come sporting the Nintendo seal in a corner (to show that it's totally not copyright infringement), and Nintendo's usually not something that touches book publishing with a ten-foot pole unless the media has anything to do with one of its games in the first place. That is, the people who are saying this probably just don't realize that this manga stems from anything thanks to the usual pattern.