The part I doubt is whether they actually shipped the SRBs via train thereby narrowing the tubes to fit thru the supposedly narrow tunnels. Heck, you can fit at least four horses side by side
(holding out my arms to approximate the width of a horse's hiney) with stall space in a standard livestock railroad car and the railcars are only as wide as the diesel engine pulling the train. And if the diesel engine can't fit through the tunnel, it's not going to matter how much smaller you make the SRBs.
Bottom line: If the tunnels, that are in the way of a bunch of rocket boosters, are too narrow, I have faith that the tunnels will get wider a lot sooner than the boosters would get narrow. Either that, or they would ship them in pieces.