The point that Irving Berlin and Woody Guthrie were surprisingly alike is a valid and important one. The both wrote simple, direct lyrics, that scanned perfectly, and could be understood, and sung, by anyone--and the point about the myth of rock roots, and it's real and tangible connection to earlier popular music is good as well--but I think the central premise, that "Anthology of American Folk Music" is taking America by a storm, while old Tin Pan alley music is either forgotten or disdained is just about the opposite of what is actually true.

There are, it is certainly true, lots of topical and ephemeral tunes the era that he mentions that are no longer remembered, but most of the songs of every era are lost and forgotten, but here are the top pop hits from "before, during, and just after the first world war"
tell me if they are "vaudeville-era pop, expunged from American music's official history" or not:

1915--It's a Long Way to Tipperary, Carry Me Back to Old Virginny
1916--Keep the Homefires Burning, There's a Long, Long Trail a Winding, I Love a Piano,
Pretty Baby
1917--Pack Up Your Troubles in Your Old Kit Bag, For Me and My Gal, Til the Clouds Roll By, Over There
1918--Hail, Hail, the Gang's All Here, Darktown Strutter's Ball, Smiles, Tiger Rag, Oh, How I Hate to Get Up in the Morning--
1919-It's Nobodies Business but My Own, I'm Forever Blowing Bubbles, A Pretty Girl is Like a Melody

It would be hard to find a piano bench the didn't have the music to at least one of those songs in it, even today. But how many people have "The Anthology of American Folk Music"?