Welcome to WebmasterWorld, thejosh!
Having done some tests, I think it is to do with "bolding" in SERPs snippets, but it appears to have some side effects to rankings. The benefit woud be to the end-user (searcher) - which is important to Google. But I do not think the content is as ignored as mentioned on Hangout. I do think however the ranking is downgraded.
As JohnMu says, it could make sense not to bold the text on the page if the text is in closed tab/expanding section. Because if the user comes to that page and searches for that text - it is not there. Cannot be seen and searching using Ctrl+F on the browser page returns no results, which may be frustrating to the user. FEW EXAMPLES:
Lets take Google Tags FAQ page you gave as an example http://www.google.com/tagmanager/faq.html
If you click on the above link, you can see that the first section is shown in the expanded mode. All others below it are in closed mode.
1) If I search for a text from the first (initially expanded) section: Tags are tiny bits of website code that let you measure traffic and visitor behavior, understand the impact of online advertising and social channels
then #1 result in SERPs is the above URL and the snippet shows the text I searched for above, bolded. This happens regardless whether I searched for the text in quotes or witout quotes.
2) If I search for a text from one of the closed sections: Redundant or incorrectly applied tags can distort your measurement and result in duplicate costs or missing data.
If I search for it without quote, it is not #1 result. On google.com the faq URL is #2 result, and the text does not show in snippet. Instead, other sentences from initially visible part of the page show in snippet. Note that #1 result is stackoverflow where searched for sentence is in "visible" part of the page (not in section that opens onclick)
If I search for the above sentence within quotes, then the faq URL is #1, but snippet shows meta description, there is no bolded sentence shown in snippet since it is not directly visible when the page loads (i.e. the sentence is visible only when the expanded section opens).
This change *may have been* perhaps implemented as a result of observing Chrome behaviour where visitors are searching for a text, clicking on the result that has this text, but hidden initially, and then using Ctrl+F to find the text on the page - and not finding it. Maybe this is why Google decided not to bold the text as a part of snippet and maybe this is why it may rank pages that have the same text immediately visible higher.
However, it appears that it DOES impact rankings to a degree - but only if the exact sentence is copied by another site that has it immediately visible. The example of this is stackoverflow ranking at #1 when the text was searched for without quotes - although the originator of this text is Google.
Which is ultimately what Broadway
said in his message above (and nicely sumarised in a much fewer words than me here!):
So, I think google does index the content, but doesn't give it the same elevated treatment as content that always shows on the page.