There are two primary issues.

First, regular identities can be linked to actual people. If someone talks about how they support AI and nanotech research in this specific context it could draw the attention of the group in question.

Second, people in this thread may be tempted to discuss whether there is any actual legitimacy to the viewpoints in question. In general, Less Wrong commentators are probably more oblivious than the most people about how frank discussions can lead to bad results even when they are being discussed in a highly hypothetical fashion. For example, having the SIAI associated with even marginal, theoretical support of terrorist activity in this age could lead to be bad results.