> fact it's not "official" means what exactly?

It means no humans in the loop. GDPNow is a tracking model designed for reading by experts.

In this case, the context is the ISM Manufacturing report recorded a surge of imports while corresponding inventory numbers are still coming in. Once they do, the model will rebalance.

To the extent the model is saying something, it’s that we need to watch inventories, particularly in construction.

It means they’re not prepared to stand behind it and stake their reputation on it.