The words in the DT version mentioned by MCP support play rather than plague - not just in that line but in the following lines which also end "ay":

Sweeney Todd the Barber, by God he were better than the play,
Sweeney Todd the Barber, "I'll polish 'em off" he used to say.

Sweeney Todd the Barber, by God he were better than the play,
Sweeney Todd the Barber, they buried him underneath the clay.

Kitty