Just for argument's sake, let's assume your right - that subconscious idea leakage is unavoidable. Now ask the question, "how did the entrepreneur come up with the idea?". The thoughts any any individual has are a function of their environment (that's the abstracted version of your theory :). There are lots of people having lots of ideas all the time, many with in similar environments as the entrepreneur. The net result is that if the entrepreneur did not allow their idea to 'leak' into YC's consciousness, some other person in YC's environment would do so within a short period of time. So there is no loss to the entrepreneur of exposing the idea to YC, aside from a small chance of a small time shifting of the 'leak' (only if they really are the first person to expose that idea to YC, which is statistically unlikely).
Simple version: even if subconscious-idea-leakage exists, the likely cost is vanishingly small, as your idea is unlikely to be new to YC. If it would have been new, then withholding it only buys you time until the next bright person exposes the same idea to YC.
If you believed in conscious/active idea leakage the numbers would look different. I feel confident that no such leakage exists - it over estimates the caching capabilities of those involved ;)