I don't follow F1, but surely with anything that's going to give a team a big competitive advantage, a.) there always has to be a line between "haven't used" and "have now used" and b.) to delay other teams efforts to copy, the later that point comes the better.

The split turbocharger was most likely identified by interested journalists at the first race. The power train was homologated for the season at the end of February, so the design was finalised prior to the third pre-season test. It's most likely that the turbo was in this form a while ago.