I've done some thinking and now I have a slightly different angle. I see that while I started out with a good intuition, it went a bit wrong on its way to conceptual crystallization.
@Fooloso4's comments are more relevant than they seemed.
In a nutshell, I think Socrates is saying to Polemarchus something like, "when you repeat that saying, you might as well be quoting one of these bad guys." And that's
even if it was in fact said by Simonides. Thus the emphasis is on an independent understanding of the saying rather than who said it, and Socrates is neutral about its factual origin.
This does still mean that his comments in praise of Simonides and the sages are ironic, but it's an irony that is more complex than I thought.
SOCRATES: Well, now, it is not easy to disagree with Simonides, since he is a wise and godlike man — 331e5
SOCRATES: You and I will fight as partners, then, against anyone who tells us that Simonides, Bias, Pittacus, or any of our other wise and blessedly happy men said this. — 335e7
If it's true that Socrates is neutral as to who originated the saying, that it's our understanding of the saying itself that matters, then of course he is open to the possibility that Simonides did in fact say it. This would make the above comments ironic, not exactly because he is sneakily associating Simonides with the injustice of tyrants while saying the opposite, but because he continues to praise Simonides, pretending to believe that he could not have been wrong, while in fact he is neutral as to the wisdom of the real Simonides. It's an exaggerated concession to his reputation, in other words, paying lip service. What matters is to question our reliance on all cultural authorities,
including Simonides.
So the targets are people like Polemarchus who ascribe erroneous notions of justice to wise people, something Socrates gets across bluntly by ascribing them instead to bad guys; and generally those who rely on cultural authorities, whether these authorities are poets or sages, without having thought about them deeply.
I came across another interesting interpretation in a paper entitled "Socrates on Poetry and the Wisdom of Simonides." The idea is that Plato is not interested in Simonides as a historical figure but is rather making him stand as his ideal poet. This is in contrast to Homer, who by this point in the the conversation with Polemarchus has already been mentioned dismissively:
It seems, then, that a just person has turned out to be a kind of thief. You probably got that idea from Homer. — 334a9
But Homer is later replaced, as not deserving of Socrates' defence, while Simonides is elevated:
You and I will fight as partners, then, against anyone who tells us that Simonides, Bias, Pittacus, or any of our other wise and blessedly happy men said this. — 335e7
Again, the crucial thing is that the real Simonides is unimportant. The new element is that because of this he can function as a blank canvas onto which Plato can project his ideal poet, in contrast with Homer, who is problematic. This is quite compelling, and it's actually sort of compatible with the first interpretation, although it does bring the ascription of irony into question (or it would make it an even more complex kind of irony). It doesn't matter what the real Simonides might have said, but it
does matter what Homer said, because Homer loomed so large in the culture, and comes in for direct criticism later in the Republic.
Notes
Futter, Dylan. (2021). SOCRATES ON POETRY AND THE WISDOM OF SIMONIDES. Akroterion. 65