How does this South African batsman survives hit-wicket despite stepping the foot on stumps?

In the ongoing match between India and South Africa; star batsman Rilee Rossouw hit the stumps while going on a back foot but was not declared out under the consideration of hit-wicket. Here’s the reason –

Mohammed Siraj, Indian pacer, who was on the bowling end, did not throw/delivered his ball to batsman Rilee to which reason, the batsman survived the huge blow and continued to bat first.