Way back when Button was with Brawn, his awesome testing times truly reflected his pace. But that was with Bridgestones that never crumbled, a smaller fuel tank (refuelling era) and fewer engine modes.
In the modern day, there's sandbags galore as the fuel tanks are bigger, the tyres can shed speed rapidly, and engine modes also make sandbagging a piece of cake. But even when there's not deliberate obfuscation of speed, it seems the mood music can be way off. Ted Kravitz was pretty sharp when he picked out Aston Martin for best of the rest after 2023 testing, but after 2024 it seemed pretty much everyone thought Haas would be a dog (including Haas).
Yet it turned out to be ok to start with and developed better than RB (their rival for most of the season).
Is shockingly bad reliability the only thing we can definitively learn from testing?