Usually, but not always, investors buy something when the price is at a point where they believe the asset is undervalued. They sell when they believe the asset has reached its fair market value or is overvalued. I haven't followed Thiel closely enough but for sure this is what Buffett is doing. Assuming Thiel is doing the same then he believes NVidia is currently at fair market value or overvalued. I would agree with the latter.