Would you say the social pressure as motivation to agree with the severity AI risks becomes significant once one voluntarily joins a community like LW?
Yes, provided you want to integrate into the community (and not e.g. play the role of a contrarian).
how many of those 500 people have authentically arrived at that conclusion by doing the math?
I don't know but I would expect very few. Also, you can't arrive at this conclusion by doing math because at this point the likelihood of UFAI is a matter of your priors, not available data.
If I were to ask the question "What threat poses the greatest risk to society/humanity?" to several communities I would expect to get some answers that follow a predictable trend:
If I asked the question on an HBD blog I'd probably get one of the answers demographic disaster/dysgenics/immigration.
If I asked the question to a bunch of environmentalists they'd probably say global warming or pollution.
If I asked the question on a leftist blog I might get the answer: growing inequality/exploitation of workers.
If I asked the question to Catholic bishops they might say abortion/sexual immorality.
And if I were to ask the question on LessWrong (which is heavily populated by Computer scientists and programmers) many would respond with unfriendly AI.
One of these groups might be right, I don't know. However I would treat all of their claims with caution.
Edit: This may not be a bad from thing from an instrumental rationality perspective. If you think that the problem you're working on is really important then you're more likely to put a good effort into solving it.