actually, he refers to this, in Q&A:

1711 -- Johnathan Swift notes the contents of London's gutters: "sweepings from butchers' stalls, dung, guts and blood, drowned puppies, stinking sprats, all drenched in mud..."

and what Swift perhaps alluded to was a sprat, (fig.)* a young, small, or insignificant person (e.g., a ragamuffin).

*in it's original sense, a sprat = a small European herring (think anchovy, smelt, grunion et al).

so what's a grotty little person (or smelt) doing in our Swiftian quote with all of that other crapola?
Monty Python strikes once again, helping us to better understand our heritage...

[a knight errant, in search of the holy grail, asks a
grotty field serf how to recognize the king]
He's the one not[E.A.] all covered in [brown]$#!+[/brown]!