"Cloaking is essentially serving different content to users than Googlebot".
I'm a user who cannot view Quora content that Googlebot has indexed even though it was a result Google said matched my query. This result is therefore unusable without modifying the URL or signing in. That is the definition of different content, HTML/CSS/JS or other semantics be damned, and a poor user experience. And that's something Google is allegedly trying to avoid.
The Matt Cutts video also talks about mobile user experiences. If I worked for Quora, I'd pay close attention as I cannot view the site at all without getting their mobile app even though, again, Google said "you should check out this Quora link for this query".
Edit: From the webmaster guidelines...
https://support.google.com/webmasters/answer/66353
•Using white text on a white background
•Locating text behind an image
•Using CSS to position text off-screen
•Setting the font size to 0
•Hiding a link by only linking one small character—for example,
a hyphen in the middle of a paragraph
`
When evaluating your site to see if it includes hidden text or links,
look for anything that's not easily viewable by visitors of your site.
Are any text or links there solely for search engines rather than
visitors?