It wasn't until I read the introduction to Naomi Reshotko's article on the Euthydemus (in Ancient Philosophy, 2001) that I grasped what Senn was getting at with his language of 'intrinsic' versus 'instrumental'.
His concern may be more accurately stated, I think, as: What is the relationship between virtue and happiness for Socrates?
As regards this question, Reshotko begins her article by helpfully distinguishing four views, which she characterizes as having roughly equal representation in the secondary literature (so that none could rightly be called 'The Traditional Interpretation'):
1. Virtue is happiness.I think Senn lumps 2. and 3. together; he counts them both as views according to which virtue is an 'instrumental good'.
(That is, to have acquired virtue just is to have achieved happiness.)
2. Virtue is distinct from happiness but is alone sufficient for the greater and more ultimate good of happiness.
(That is, to acquire virtue implies acquiring happiness as well.)
3. Virtue is distinct from happiness but necessary as a means to happiness.
(This is the point, I gather, where some commentators have wanted to say that virtue is merely an 'instrumental' good.)
4. Virtue is distinct from happiness, but happiness is not a good, and virtue is indeed the sole good.
Senn seems not to distinguish between 1. and 4. In some passages he seems to argue for 4., as in his conclusion:
...as I have shown, there is according to Socrates a certain condition of one's soul that is the only thing intrinsically valuable for one.And yet in another, curious passage, Senn seems to favor 1., since he seems to think that this 'sole intrinsic good' should be referred to also as 'happiness'. After Senn discusses the two key passages in the Apology where Socrates suggests that others cannot harm him (30c-d, 41d), he remarks:
It seems reasonable to conclude that, according to Socrates, some have a happiness that cannot be taken away even if all they possess is virtue.And then in a footnote to this sentence he attempts to justify his introduction of the term 'happiness':
To the typical ancient Greek speaker, good and bad, benefit and harm, are thought of in terms of the 'happiness' (eudaimonia) of the agent in question ... Though Socrates does not in the Apology passages cited use the term 'happiness', there is good reason to understand him as having this in mind.But this is just to drift without argument from 4. to 1.