concessive or still causal?

Which of (2a) to (2c) do you think explains what (1) means?

(1) He is not so busy that he can’t go for a walk.

(2) a. He is very busy but in spite of that, he can go for a walk.
b. He is not very busy, but that doesn’t mean that he can go for a walk.
c. He can go for a walk, but it is not because he is not very busy.

Thank you in advance
Seiichi MYOGA

My question is based on the fact that (3) means (4) (or at least Swan (1995:513) would say the same thing).
(3) He is so busy that he can’t go for a walk.
(4) He is very busy and [color=red]because of that, he can’t go for a walk.

What I want to know is the perceptual influence of the inserted “not” in (1) on the implicit causal relation found in (3).

The meaning of (1) is (2a).

Dear Jamie (K),

I appreciate your help and comments.

Seiichi MYOGA