Rainbow Tables are a precomputation "time-space tradeoff" attack. You do a bunch of preparatory work which is amortized over multiple attacks and results in needing space to store all your pre-computed data. This is nice for two reasons:
1. You get to do all the hard work before your attack, leaving less time between the attack and your successful acquisition of the passwords compared to work that's necessarily done after stealing the credential database.
2. You can re-use this work in other attacks
But if you're waiting until you know the salt you don't get either of these advantages, so Rainbow Tables are irrelevant.
It's like if somebody mentions the F-14 fighter jet in a discussion about the fastest way to get from Times Square to Trump Tower. Yes the F-14 fighter jet is a fast aeroplane, but it can't go to either of those places so it isn't relevant whereas Usain Bolt is a very fast human so he really could run from one to the other.
Rainbow Tables are a precomputation "time-space tradeoff" attack. You do a bunch of preparatory work which is amortized over multiple attacks and results in needing space to store all your pre-computed data. This is nice for two reasons:
1. You get to do all the hard work before your attack, leaving less time between the attack and your successful acquisition of the passwords compared to work that's necessarily done after stealing the credential database.
2. You can re-use this work in other attacks
But if you're waiting until you know the salt you don't get either of these advantages, so Rainbow Tables are irrelevant.
It's like if somebody mentions the F-14 fighter jet in a discussion about the fastest way to get from Times Square to Trump Tower. Yes the F-14 fighter jet is a fast aeroplane, but it can't go to either of those places so it isn't relevant whereas Usain Bolt is a very fast human so he really could run from one to the other.