"When attacking, during the "Modify Attack Dice" step, you may receive 1 ion token to choose 1 of the defender's focus or evade tokens. That token cannot be spent during this attack."

During the modification of your attack dice or the defense dice of the defender? So I am not allowed to wait for the result of the defense dice?

You answered your own question; 4-LOM says during the modification of your attack dice.

As you roll your TLs/spend focus etc.; based on the final result of your modifications, you can tell your opponent that he can't use a focus/evade when he goes to defend.

So, for example, you roll a 4 dice attack and all are <booms>/<kabooms> and your opponent has 2 AG with a focus & evade token. You could tell him that the evade token can't be spent; meaning at best he can only roll two evades.