You've forgetting the most important factor in whether a radar can detect a target or not. Range!

If you halve the range to a target, you increase the received radar reflection strength by sixteen times!

If you quarter the range to a target, you increase the received radar reflection strength by 256 times!

Basically the SDB has to get close enough to the S-400 that the SAM site won't be able to react in time to the SDB before it hits the site. I don't know for sure how fast SDBs are, but I guess they are slow, they're just glide bombs. So maybe 200m/s. So even 4km away would give the S-400 twenty seconds to react. At 4km the radar reflections of even a small stealthy target are going to be huge and easy to detect.