The pot life of thinset varies depending on the type, how it is mixed, the humidity, and how damp the surface you apply it to. If the thinset still mushed when you screwed it down, maybe. If it were tile you were trying to adhere, absolutely not, but the goal of thinset under cbu is to fill in minor imperfections so the board is fully supported. IF that happened, you should be okay. Otherwise, you've got problems. Hard to tell without being there. No guarantees. Safer to remove and replace, but it could be fine. You'll get more and better results where you started this over at www.johnbridge.com - those guys deal with tile every day.