I think Rothbard is not clarifying Mises there; he is contradicting him, and I think he is making a mistake in doing so.
I would say (and I think I am following Mises) it is the proportion between consumption and non-consumption (hoarding + investment) that reflects time preferences.
Again from Human Action:
"He who consumes a nonperishable good instead of postponing consumption for an indefinite later moment thereby reveals a higher valuation of present satisfaction as compared with later satisfaction"
That's time preference, and there is no need to invoke investment or interest to explain it.
If the stored coconuts keep Crusoe alive while he builds a boat...then what? They provide all the same present utility prior to consumption. Are they now an investment rather than a hoard? What's the difference?
There is no obvious distinction between saving and investment in an autarchic economy. There is always time preference though.