It is agreeing with the case of the subject, as nouns and adjectives do after verbs meaning be, appear, become, be thought, made, named, chosen, regarded, and the like (Pharr 974). That is to say, the thought of Agamemnon is being identified, in the strictest sense, with the subject. The case usage follows precisely the manner after which Homer has Achilles say in the first Book, "δειλός τε καὶ οὐτιδανὸς καλεοίμην, | εἰ δὴ σοὶ πᾶν ἕργον ὑπείξομαι".
The νόος of Agamemnon is not his mind, but what he has in his mind, the state of action and obedience he desires of his men; the lines do not well translate to what is just the English idiom, "For thou knowest not yet clearly what is the mind of the son of Atreus" (Murray), because then "mind" would be in the accusative as the direct object, as it is in lines 262-263 Book1. To this would be objected that the verbs are different: the example from Book1 uses the verb "to see", and perhaps the perfect "to know" just happens to adopt the predicate-identity usage as a peculiarity of Homeric grammar? I cannot say I know for sure, but I doubt this is the case, having respect for Homer and his logic. But I can say with assurance that the perfect meaning "to know" naturally follows as what has been seen; but the perfect after all is still just the perfect! The perfect describes states. Hence this interpretation: "Clearly you are not realizing the sort of Agamemnon's intention" = "you are not being the kind he wills (you to be)". This agrees better with the note Monro made regarding line 190: Odysseus conveys delicate exhortation; and he does so all the way through to the end of line 194, before finally bringing to mind consequences, appropriately and in a logical manner.
This interpretation though subtle is significant in appreciating the web Homer spins.