In my opinion not attending this event is not only a hint to stealth mode (or at least to a somewhat "stealthy" mode), but in the first place it is a sign of being extremely focused on a single target. I believe they considered this conference as not being as suitable as other measures to achieve this goal (or not suitable at all), so they decided to not attend. This what setting priorities means.

A reason could be that those who should know about POET at this time already do, so there would be no point (= immediate benefit) in creating a wider audience. I believe when they will release the news about what they are doing and with whom, the word will spread quickly and widely anyway.