There's no doubt Sobers was initially picked with bowling principally in mind, much more so than batting - however early in his career, his bowling was undeniably ineffective. In his first 21 Tests, he averaged just a tick under 52 with the ball. No amount of "it's not all about statistics"ism will change the fact that this = ineffective.
However, Sobers' batting wasn't of any particularly huge note early on either. As already noted, he was first picked at the age of 17. Few, even the very best, will be able to perform in Test cricket at this age. It took another 4 years until Sobers' batting begun to come-off. In his first 14 Tests over those 4 years he averaged just 30.54.
The true reflection of Sobers should only start in 1958. For the next 11 years and 56 Tests, he averaged 71 with the bat. This truly is mind-blowing. Virtually no-one has ever managed this. This is why Sobers has more than legitimate claim to being the second-greatest batsman after Bradman.
However, as a bowler, Sobers was still to start being effective. Even starting in 1958, he still retained an average of over 50 for his next 20 games. It was only midway through the famous 1960/61 series that Sobers finally began to show the bowling skill he had been selected for. For the next 8 years and 33 Tests, Sobers averaged 28 with the ball. Only midway through the next series in Australia did Sobers' bowling begin to decline again, and in his last 26 Tests he averaged 37 with the ball.
Only for a portion of his career was Sobers the phenomenal all-rounder we know him as - but this is still an achievement no-one else has come close to matching.