That's because every colour and gamma value will be 'wrong'.
Not just the extremities.
You must remember that you are effectively 'squeezing' all the P3 image colour and gamut into Rec709.
This makes me wonder. If I am planning to deliver for Broadcast (Rec709), why would I consider shooting Slog3? It seems that much of the color space must be discarded, since the delivery format has no use for it. In a sense I am wasting a good part of the data that I record, because I have no use for it. Am I correct on this? I can see the value of increased bit-depth, but capturing wider gamut seems pointless if you have no possibility of ever displaying it.