Just to respond to 2 (I disagree with 1 and Ed already extensively covered in another comment), I agree with you! It primes the model for the task, which might be more fair than zero-shot as a comparison to humans who are otherwise motivated, we do not currently know (e.g. see Andrew Lampinen's paper on the topic https://arxiv.org/abs/2210.15303).

We argue in the paper, and here, that ~5% is significant, and additionally on the subset of particularised examples the different is ~9%. The actual significance of this gap in terms of whether this will be noticeable to humans in some kind of Turing-style comparison is an endeavour for future work. I personally expect it to be first of all noticeable and second of all, to re-iterate, this is a very simple type of binary conversational implicature; it remains to be seen how they fare on more complex implicature.