Sinclair looks at Jack with mild surprise, but only for a moment.

"Of course," he says, regaining his composure. "We were just on our way back to Olympus Heights. It's safer there than out here in the middle of all this. Why don't you join us?"

...Not that he doesn't trust Jack to conduct a conversation on his own, but this is their escape plan that's at risk here. If Jack says the wrong thing, they're done for. This is the only plan he's got right now, if this gets messed up then they're out of luck. And that's can't happen.