I have text in the meta description field of Basic Board Information, but does not show in the page source
Yes, well it certainly caught me out.Maybe the description text is slightly outdated for that option.
It's not necessary because what you add in Basic Board information shows there. My only issue was that the implication of thew wording was that it would show on home page.Nonetheless if you want to add the meta description to a template the forum_list template should provide some clues how to do that.
There may be a delay before changes are rolled out to the XenForo Community.Slightly adjust the explain text for the boardDescription option to clarify it applies to the "Forums default page".