I worked McD in the 80s and the Coke syrup was in gallon jugs at ambient temp, then dumped into a large stainless steel holding tank at pressure. This was before bag-in-box. I've never heard of the cold temperature thing before.
My belief is that they just consume so much of it so fast that there's no time for the syrup to alter.
https://youtu.be/GA_OYUHYYMA?si=B-EXCoGyr9JGq87n
Coke tastes different at McDonald’s because they keep the syrup cold from the factory to the restaurant.
Why? Because it prevents the syrup from fermenting.