Earlier quoted context omitted.
Right, because it's much harder to measure a specific amount of milk in isolation than to eyeball how much of it you've just added to something else. If you were concerned about regulating the amount of milk you were adding, tea first wouldn't even be a possibility.
The line where he points out that you need to stir continuously is important. He's saying that by mixing the milk and tea reliably, you can eyeball the colour of the tea. Indeed, this helps you regulate the flavour over multiple cups as the tea may be stronger (and therefore darker) if you have not removed the tea leaves. Conversely humans are terrible at measuring volumes in isolation in a container thar may differ…
Most people use their teacups more than once.
Even among people who don't, disposable cups, having been mass-produced, are all the same size.