Bro. Stone wants to emphasize the aspect that his sacrifice was for others, to which we do not object. What we object to is that he completely ignores the fact that the atoning sacrifice itself, was also for himself. The verse is quoted to him, because all Christadelphians (even bro. Stone as we shall see) understand how this verse should be translated. When Paul speaks of Jesus "having obtained eternal redemption", the King James translators added "for us" to the end. This is italicizedindicating that it is added by the translators. But it is more than just added. It is an incorrect translation. The actual translation should be "having obtained eternal redemption to himself."
The Greek word is the verb "euramenos". It is in the "middle voice" which we do not have in English. We have the active voice, where you do something, and the passive voice, where something is done to you. But Greek has a middle voice where someone does something to oneself. The is the state of this verb in Heb. 9:12. So the redemption obtained in his blood, is obtained in and for himself. Bro. Roberts makes this argument in "The Blood of Christ", and recommends this as the correct translation: "Having obtained in himself eternal redemption." So yes, the sacrifice was for us. But this verse is also describing with great force, that it was for and to himself, as well.