There are the explicit guidelines, there is common courtesy, and there is outright lying.
If Yale made offers to external candidates last year prior to the set date, that may have violated the guidelines, though the way the guidelines were phrased this year, it would have been appropriate to say "we plan to give you an offer."
If a candidate at any point tells a program that they are going to accept an offer but they have no intention to, that is lying, and against common courtesy.
This is a field concerned with semantics and reasonable behavior after all. The grey zone that I encountered this cycle was that I had told a program they were my first choice prior to Sep 16, and then they didn't give me an offer in the first 24 hours, and personally I no longer felt beholden to the statement I had made once I received other time-sensitive offers.
Again, my conclusion is that this headache could all be avoided with a match, especially as the field becomes more popular and competitive.