Instead Jupiter should command Achilles to go to Ajax.

Exactly.

The whole thing is a performance. Ajax is the audience. Ulysses is the director. Achilles is an unwitting off-stage actor, predictably playing his part.

I don't recall if Agamemnon and Ulysses had discussed sending Ajax to talk to Achilles himself when plotting the scene earlier. As a director, if there was no explicit prior discussion, I'd stage this as Agamemnon trying to direct the action. But Ulysses sees that it would be a big mistake to actually have Ajax and Achilles meet, so he improvises an over the top excuse to justify blocking Ajax. Notice that Ajax talks about fighting Achilles. That would be a terrible outcome for Ulysses. The goal is to get Achilles to fight Hector. Ajax is just a tool, and the whole speech is there to manipulate the big dumb warrior. (Part of the humor is that Ajax is too dumb to be easily manipulated because he doesn't understand what's happening.)

If there was a reference to Agamemnon sending Ajax into the tend as part of the plan, then the staging is similar but without the improvisation. Instead Agamemnon is giving a cue Ulysses to go on his speech about Achilles waiting on Ajax instead of the fool Ajax waiting on the hero Achilles. This is a bit less exciting and lower stakes than the improvisation though.

Putting the command in Jupiter's mouth shows how extreme the overstatement has become. It's a big performance. So many Shakespeare plays are meta-theatrical, and Troilus and Cressida, probably Shakespeare's most complex and secretly best play (IMHO), the acting and directing is crucial to understanding the play. The idea that the war itself, the heroes and myths of the Golden Age of Homer are all lies and distortions is explored and driven home by all the way the characters themselves act. From Achilles and his butt boy's satirical interludes to Cressida's performance of feminine aloofness to Helen's face being painted with the blood of dead soldiers, everything in Troilus and Cressida is a performance. Achilles' victory over Hector is a lie and his myth obvious propaganda spread by his Myrmidons.