He doesn't mention JW's directly but does mention the Millerites.
No, Sinclair doesn't mention the JWs because they were known as Bible Students at the time he wrote this essay.
In Book 6 of The Profits of Religion, in the chapter titled Bible Prophecy (pg 203) Sinclair speaks of Pastor Russell and the Bible Students.
Upton Sinclair was spoken of favorably in the Golden Age magazines during the years that the WTS was pro-Socialist.