Searching google books for “reading slop”, this usage seems to be very sporadic (only 11 hits for 20th century publications), I’d think it was more just a case of independently coming up with the same shorthand rather than causal influence, whereas slop seems to have been used this way frequently enough on 4chan that it seems much more likely there was an actual chain of influence to later use on twitter and elsewhere, especially since a lot of other 4chan slang has later broken out in that way.

I think this may have been a case of the “dirtbag left” types using slang that originated on 4chan, I did a search with date restrictions on the 4plebs archive site and found posts using it for mindless entertainment going back to 2012 at least