Imran averaged 32 in 1980 and 27 in 81. This isn't test standard. Then he missed 3/4 (or whatever) of games in 84, 85 and 88. Plus could not bowl.Imrans bowling peak from 80 to 88 was around 48 tests and he averaged 40 with the bat then. He then played 15 tests as a pure bat towards the end, averaging 72.
Sobers from 61 onwards took over 3 wickets a tests for around 60 games, coinciding with his batting peak.
Kallis had around 60 odd tests in the beginning taking 2 wickets a test, and as he took over a SAs main bat, his bowling output decreased to 1.5 wickets a test for 100 tests.
So Kallis compared to Imran and Sobers wasn't test class standard as a bowler while batting for the majority of his career. That is the major difference. As his career progressed, his bowling diminished, whereas with the other two, their second discipline was still doing strong.
Here's Sobers from an article. The 69 onwards looks a whole lot like Kallis mid to late 2000s.
As I said, very comparable timeframes.