Here's what Dennis Alexio had to say when asked about it. Unless you've been living under a rock, Dennis was in the film Kickboxer with Van Damme.

"Jean Claude is not and never was a kickboxer. Jean Claude told me he had one fight and his trainer was Dominic Valera. I spoke to Dominic and he told me Jean Claude never fought full contact and never trained to fight in the ring. On the set of Kickboxer we tapped around and played tag and he has good flexibility but is not a fighter by any sense of the word. He is capable to a degree but not on a world class level or even a competitive professional level"

I also have an article where Van Damme claims he fought for the world middleweight kickboxing title (in the FC days) and knocked out the world champion in 30 seconds with a head kick. News to me!