Well...first we need to consider rule Nr.1: Never believe what Toto says.
Then...I mentioned it in the RB team thread when we discussed the engine penalty there. Merc is marginal with loosing the FP1 and 2 engine and just two engines left. My clear suspicion is, that RB can take a "cheap" penalty if they are anyway well behind Merc and maybe even behind McLaren. Just some few point loss for the 10 place penalty.
The new engine would put them immediately on a better position than Merc, as the milage in sum on the two engines would be way less than what Merc needs to do.
Now if we see this from the Merc perspective...why not cover this move?
If they are well ahead of RB in Q and maybe even well ahead of the rest of the field...why not go back to P10 with Ham and see where they end up with him. There is only Lando to keep him from P1 if my prediction is correct. The rest is either underpowered or a "friend" like Russel. As long as Ver is going back to P13, no harm is anyways done and he has less traffic to clear. This would give a good engine situation compared to Ver in the end and not put them on the back foot.
In this regard I expect no one to announce the engine swap before the last minute before the race.