Earlier quoted context omitted.
Apparently milk before tea is lower class because poor people would want to avoid cracking low quality porcelain with hot tea. Fascinating!
Was that a genuine concern? Boiling water has a fixed maximum temperature. Not sure that room temp->90C is all that different from room temp to 100C for ceramic.
When you pour hot tea into an empty cup, the innermost layer of the cup goes from room temp to ~80C within a fraction of a second.
When you pour hot tea into a cup that already contains a fair amount of milk, it goes from $milkTemp to ~60C gradually over a few seconds as the tea mixes with the milk, giving enough time for the cup to expand evenly.