I think they needed a prep lap to get the softs working, but because they expected rain 3-4 mins from the end, they fueled the first run such that they would end up pitting just before the rain (like everyone did). But that meant when the rain didn't show up, there wasn't enough time for a prep lap in the second run, only a warmup.
They were right with Merc/amr until the final run when they had to fire the tyres up immediately. Further supports the idea from Jeddah that they struggle for tyre temperature?