They do!
The Merc system monitors the battery voltage drop during cranking. If the voltage drops below (IIRC) 11.5V then it disables Stop/Start. It'll still start all the way down to less than 9V.
It's not really the state of charge that matters, but the batteries internal resistance. The resistance increases as the battery ages, so it can be fully charged (>13.7V) but still drop enough during cranking to disable the Stop Start.
Mine's been like it for over a year. Behaviour confirmed by iCarsoft scanner. New AGM Stop/Start battery would no doubt cure it, but why on earth would I want to do that! £300 to re-enable something I don't want.
If they do then they are fools! (I am confident they don't as we use the same setup!), they wouldn't meet ISO 16750-2 under those conditions
Monitoring the terminal volts under load tells you little about the battery condition as there are to many variables, temperature being the very significant one, as the chemical reaction in the battery is much poorer at low temperatures and so the VBatt drops much lower. There are also variables with connector/cable ageing, engine types and a whole host of other things.
The SoC is the critical measure for start stop, as it is the only means to tell if you have the reserve to complete repeat re-starts, its also the only viable means to determine battery health by checking the batteries ability to absorb energy