This one is unsolved. See below for why I looked this up.
Turns out there are a lot of Frank Leslies. I’ve researched every one of them, and I can’t find one that matches the reference.
The closest I can come is a magazine publisher that perhaps published some consumer advocate stories, but I can’t find any reference to him having been popularly known for that.
On October 26, 2021, I contacted Steven Johnson and asked for clarification. I am awaiting a response.
I’m leaving this here in the hopes of a future resolution.
Why I Looked It Up
In Extra Life, Steven Johnson discussed Ralph Nader:
Nader’s key contribution lay in inventing a whole new kind of political figure, a Frank Leslie for the television age.