Senn's first argument for his thesis is a fallacy; but the second needs to be taken more seriously.
Here's the first:
There are certainly a number of passages in the Apology in which the good condition of soul is characterized as the greatest good. Consider Socrates' habitual exhortation to everyone 'to attend to neither bodies nor money before--or as vehemently as--[you attend to] the soul in order that it will be best' (30A-B; cf. 29D9-E2, 36C, 39D). He also uses the word 'virtue' to describe that which he exhorts everyone to attend to (31B, 41E; cf. 29E5, 30B3). Socrates' final exhortation in the Apology (41E) makes it clear that he thinks there is nothing that one must attend to before attending to virtue.But of course this is a fallacy, because from 'virtue is a greater good than external goods and the good of the body' it does not follow that 'no good is greater than virtue.' (And that conclusion even more clearly does not follow from 'attend to no good before virtue'!)
Senn seems not to recognize the fallacy. His next sentence is: "None of this, however, proves that for Socrates virtue is the sole ultimate end", which might suggest that Senn does see the fallacy. But, if he had seen the fallacy, why would he have written, "There are certainly a number of passages in the Apology in which the good condition of soul is characterized as the greatest good"? Also, the fallacy consists in concluding that virtue is an ultimate end, not that it is the sole ultimate end. (Is this a place where Senn gets into trouble from conflating ultimate and intrinsic?)
In any case, Senn puts this argument aside and turns instead to his second argument, viz. that Socrates' remarks about his immunity from harm make sense only on the supposition that he regards virtue as 'the sole intrinsic good'.
But the fallacy is worth noting en passant because it is probably not uncommon.