Looking back at the lap times of Riccardo, who knocked out 43 laps on his mediums and was still running the neigh on similar lap times down to the tenth/hundredth of a second compared to the early laps and the end laps on his medium stint.
From that photo we can see that the softs of Max were starting to drop off by .750 between laps 16/17 and lap 24. I think at that point RB knew that the medium was dropping in terms of lap time and made the right call to pit max and checo.
What is even more BAFFLING, is the reason Toto said the hard was a good tyre because Latifi was doing well on it. (according to a Sky Germany interview).
Lets look at Latifi and his swap onto the Hard tyre..
Lap 25 was the first run on the hard for Latifi. For reference Hamilton pitted on lap 30, giving a 4 lap data gathering from Latifi's hards for Mercedes to make the call....
latifi was doing a 1:25.4 on lap 17, losing a tenth each lap up to lap 20 where he ran a 1:26.661 . Lap 21 and lap 22 seemed a bit of a outlier with a longer time with a 27.1 and 26.4 respectively. So not sure what happened there whether there was traffic of a tyre concern.
Then it gets interesting when he swapped to the hard...
Lap 25,26,27 were a 1:25.078 and within a few hundredths of each lap on the hard with a 1:26.4 on lap 28. So Mercedes literally had 3 aps of hard tyre data in order to make the call that the tyre 'looked good' on Latifi's car. So the fresh hard on the Williams was 1/2 second faster than a soft that had run 22/23laps. That's hardly impressive delta on the tyres in order to make such a bold call like that.
Hamilton's tyres really didn't seem to drop off either, averaging low 1:23s and only marginally slower when Max pulled a little bit further away to clear the tow for Lewis. Once Lewis got back into Max's tow the times were exactly the same as he was doing early on. So I think the call that Toto claims for the swap is trying to be damage limitation for the fact they screwed the strategy up. They were probably more concerned of the fact that Max was 0.75-0.9 seconds faster on a fresh soft that their old medium on 28 laps. That difference is nearer to 1.1 seconds with Perez who pitted a lap earlier than Max.
Edit: Apologies if the pics appear small - they are the correct size if you click on them
Mess with the Bull - you get the horns.