He would have known, via Wanda that it is possible for the Mind Stone at a minimum to be destroyed. All it takes is one single stone to be taken off the board and the plans of Thanos are at an end. Since they have the Mind Stone at that point he's in a position to make that happen, otherwise if not they could always go with the losing it in the quantum realm plan. Tony gets Hank onside with him in record time by helping him to rescue Janet so that's an option. The only loophole might be the potential use of the Time Stone to wind the clock back like Thanos did originally, however Tony would be aware of the Ancient One via Banner who would also be in a position to help out and possibly lay the grounds for Strange to later lose or destroy the Time Stone after he's used it to defeat Dormammu. With their ability to use the stone to view possible futures, doing that at an earlier point in time might open up additional paths to victory via conventional means.

Of course avoiding the snap may simply end up bringing The Emergence forward to happen sooner and with Tony unaware of this it may end up being the end of Earth anyhow. Strange may have been taking a much longer term view of things.