True, Timothy - it is a big claim. But consider the following:
Most hundreds in a career
Kallis currently sits in second place, with a higher average than Tendulkar (as well as most of his competition). Tendulkar is considered the greatest cricketer ever by some - certainly the greatest batsman of the modern era, with Bradman his only competitor in terms of all time greatest batsmen.
Now, what is evident from the above is that if Kallis had never bowled a ball in his life, he'd be one of the greatest test batsmen ever. But he has. He's taken over 270 test wickets. To put that into perspective, that's more than the likes of Jason Gillespie and not far behind Brett Lee (remember Kallis is still playing).
This article from last year is a must-read for any cricket/stats lovers: The Numbers Game: Jacques Kallis, take a bow
This is good too: Kallis is the Sobers of his generation
As an aside, while this proves nothing, I thought you'd be interested in reading two quotes from your beloved Kevin Pietersen:
"I truly believe that Jacques Kallis is the greatest cricketer ever. He's just phenomenal." - from an interview.
"Kallis scores his 36 test ton.. Along with his 250+ wickets & doesn't drop a catch... Is he the best cricketer ever??? I think so!!" - from Twitter.