This is Michael's objection to Harry's plan. In terms of how and when the Knight's act it is often communicated to them in some fashion. This is discussed over and over again in the books. Michael and the other Knight's are where they need to be when they are needed. They work for a god.
...and Sanya's plane being "coincidentally" delayed wasn't enough of a sign for them? (Thanks for the quote, by the way.) The more I think about this scene, the more issues I have with it. Possibilities for why it played out the way it did that I can see are:
1) Michael distrusts Harry (which he admitted) ... and yet goes along with Harry's second plan rather than waiting for him to leave before planning a rescue mission with Sanya.
2) Michael objects to Harry's plan, but still trusts him enough to let him lie to the White Council (which it has been pointed out to me is out of character, and furthermore Michael admits later in the book to thinking Harry has been subverted by Lasciel).
3) Michael doesn't think they should interfere, despite "coincidence" indicating that TWG wants them to, but can't let Harry face the Denarians alone (the former part seems
highly problematic; the latter part has canon support).
4) TWG has indicated to Michael that he should not go after the Denarians directly
despite the fact that it would be a rescue mission, but has not expressed an objection to Harry's second plan (which...did He
want Ivy to be tortured? Was it some twisted way to see to it that Ivy would get more free will than she would otherwise have with the Archive? Is getting Harry access to soulfire that important? What?)
5) No one cares about Marcone. The Knights are only responsible for rescuing innocents (possible. In Skin Game, Michael says he can't help Hannah at the Gate of Fire because she's not an innocent, that she chose to put herself in danger. On the other hand, the Knights' explicit job is to save the Denarian hosts from the Fallen, so they can't
only be interested in innocents).
6) Everyone spontaneously, inexplicably
forgot about Marcone (which seems poor writing--there ought to be a reason).
All of these, to my mind, are problematic. (Another possibility is that Harry never told the Knights about Marcone--I don't remember whether he did or not--which is still problematic, but is not a problem for Michael, it's just Harry being inexplicably more idiotic than usual.)