This looks to be the same size as my brother's 650C QR Kilo which is a 53. He is also 5'6". Triathlon bikes often use a "dropped" top tube to get the handlebars lower for a better aero position and the frame is usually measured center-to-top of seat lug.
Looks like a nice ride. If it were mine, I'd re-route the shift cables over the top of the base bar between the armrests. That angle on the rear shift cable housing at the downtube cable adjuster just looks wrong.