Wordpress is a CMS (content management system). Its not the forum.
You add “plugins” to wordpress to extend its functionality.
This site seems to be using BBPress for the forum plugin and it seems there is no preview functionality build into BBpress.
Thanks, after your comments I did a bit of digging, BBPress is a plug-in from WordPress; no wonder I am confused.
Anyway, looking about in the BBPress forum I see folks asking the same question I am. And, you appear to be correct, no preview capability.
That forum talks about “fancy mode” for this post editor, which is WYSYWIG.
That would help, but probably slow down the Synthetos web server even more.