You can't have a disposition to act in a certain way without counter-factually acting that way. You can't counter-factually act a certain way without actually acting that way in a situation indistinguishable form the counter-factual.

What is the relevance of this? Are you using this argument? (See comment above.)*

1. It is impossible to have the one-boxing disposition and then two-box.
2. Ought implies can.
3. Therefore, it is false that someone with a one-boxing disposition ought to two-box.

If that isn't your argument, what is the force of the quoted text?

At any rate, it seems like a bad argument, since analogous arguments will entail that whenever you have some decisive disposition, it is false that you ought to act differently. (It will entail, for instance, NOT[people who have a decisive loss aversion disposition should follow expected utility theory].)

Notice that an analogous argument also cuts the other way:

1. It is impossible for someone with the two-boxing disposition to one-box.
2. Ought implies can.
3. Therefore, it is false that someone with the two-boxing disposition ought to one box.

*I made a similar comment above, but I don't know how to link to it. Help appreciated.