You can only say that if Midwestern farmers ignored the railroads and persisted in trading down the Mississippi with New Orleans. Did they? I've yet to see the evidence. Clearly, before Vicksburg fell, the Midwest had to use the railroads. Did they switch back to river traffic south after Vicksburg fell, or did they continue to use the railroads east?Indeed, but it seems that's now more apparent to us than it was to them.
Or is the "them" referring to Winfield Scott who was 70 years old when he came up with the Anaconda Plan and may not have understood how those newfangled railroads were going to alter traditional trade networks.