I have a hard time though believing, that talking at a big conference like that, could "slip under the radar" regarding MB and his calendar. Such an event must be coordinated with NWBO especially if it is depending on journal having been published beforehand.
When they first registered a long time ago, they knew they were going to speak about "trial challenges". Topic generic worded. But since we got the "coming months" signal some months before the registration was public in beginning of September, they MUST have believed, they would be OUT of the quiet period, when the conference "hit".
To do the talk, MB needed for TLD and publication to have happened, in order to speak about the challenges.
He cancelled very late. Two days after he should've gotten the reminder about him being pushed. One can read something into that, and one can choose not to. I believe though, that it (TLD) remained a possibility and they left the "door open", just in case and ended cancelling very late.