I think the point they are trying to make is that sometimes less information is better. For example, if I am reading an article about dog walking (example), I don't necessarily want my mind to be distracted by the wikipedia link to Wheaten terriers. This type of information, when presented as Readability does, is better handled as a footnote.
There are many sites online that I prefer to read using Readability (Gruber comes to mind), but there are more that I prefer to read in the normal hyperlink format we all know and love. For the ones where I use Readability, I think the option to use footnotes is nice.
footnotes are convenient when reading a physical object because, as you imply, more information right there can be distracting. however if you choose to go find the footnote, that's up to the reader and would objectively qualify as a distraction. similarly, a link with no hover over text is left for the reader to choose to click or not. this IS a footnote in a sense and is the modern paradigm used for digital reading.
I think, having a link in the middle of a sentence is more distracting than using a footnote style link at the end of the sentence. EX:
The war of 1812 was started by the United States in response to a series of trade restrictions (do you want to know more?) to impede on-going American trade with France. Vs.
The war of 1812 was started by the United States in response to a series of trade restrictions to impede on-going American trade with France. (do you want to know more?)
I often find when I am reading Wikipedia or tvtropes.org just processing all the great links takes significant effort. AKA why do I have 15 tabs open.
So my text (would you like to know more) is trying to represent that choice. And my point is the presence of a link or a footnote both represent the option to learn more and when creating content deciding where to place those links is important.
PS: Wikipedia uses both and I find the footnotes are far less distracting.
There are many sites online that I prefer to read using Readability (Gruber comes to mind), but there are more that I prefer to read in the normal hyperlink format we all know and love. For the ones where I use Readability, I think the option to use footnotes is nice.