At the bottom of p. 210, it says "smart recommends an antifreeze/corrosion inhibitor concentrate in accordance with MB Specifications for Operating Fluids 310.1 or 325.6" The products for 325.6 are European and not easily available in the USA. https://bevo.mercedes-benz.com/bevolisten/325.6_en.html
Going out on a limb here but I'm guessing the same coolant that worked for the 451 would be OK for the 453 gasser. I'm also confused by the "which only applies to cars manufactured until April 2014" wording. Since the 451 carried on here for the 2015 model year I don't think the change applies to our final model year 451s. YMMV.
The required coolant is likely used in other Mercedes models. When I changed the coolant in my 2009 451 I went to the local (non-smart) Mercedes dealership and got a gallon of 325.0 factory spec coolant concentrate. Beats trying to determine if the aftermarket stuff is correct and the price was very reasonable at $23.50.
P.S. Did you see this note?
3. Mixing Ratio
3.1. Sheets 326.x
The products listed in the Specifications for Operating Fluids 326.x are coolants or finished mixtures and must not be mixed with water. When using them, compliance with the concentration (50/50) must be ensured in addition to the MB approval (see item 5).
In this thread, I should've used the term "antifreeze" instead of "coolant."
I called the service center to order 325.0 MB antifreeze for my 453. The representative looked it up and said that 325.0 is not for the 453 model. I'm going to wait until the next time I can go in person to see what the MB antifreeze for the 453 smart car is.