The way that I have seen color variations in text (black to grey) used on sites has been primarily to help the end-user visually sort the Information Architecture of the content they are trying to digest.
This is commonly seen on forum boards where the post text is of a dark nature (usually from around rgb 0,0,0 to 70,70,70) as a means of saying "this is what you should be looking at". Then you have secondary information, sometimes user quotes, sometimes control options, all designed to be 'visually' on a 2nd level where if you were to want to "do" something, you could then focus in on this information. So basically, it's designed to help the user discard the information that they don't need right now (or quiet the interface's clutter/noise, lessen cognitive loads), and focus in on it when they do need it.
Here on Stack Exchange you'll see common engagement options on questions, for example share/edit/flag, or post time, or a control to add comments, or other actionable items all lighter shades of grey than the primary text color.
As for Patreon, I do think that their usage is a mistake as when structuring a Q/A section, Q&A are undoubtedly a primary piece of information in that content structure. Based on their usage I think they are trying to separate larger-scaled categories (common questions, privacy, etc...), but ultimately a margin change would do exactly what they need and would be less problematic. I say problematic because one drawback I see on a first glance is that the twitter feed draws my attention more than the questions do solely on the saturation of color/darker text.
One last tidbit to gnaw on, another somewhat common usage for lighter greyed text is when user controls have been disabled, but as you would quickly ask, how and what level of hues/s differentiate secondary actions and disabled controls? Well, yes this could become tricky and confusing to the end-user if not handled correctly.