There are two reasons they didn’t dose him up on adderal for his depositions/interviews.
1) They have to try to keep him off the stuff as often as possible so that he doesn’t build up a tolerance/dependency for it and thus become even more incoherent when he is off of it (hence why he very rarely does interviews and events).
2) They probably calculated that having cognitive deficiency for these interviews might be advantageous. He wouldn’t have to feign being forgetful. It would be a genuine response.
And they were right. Hur didn't prosecute for that reason.