Confusing statement in part 5

[corneliusroemer]

I love the tutorial, but this doesn't make sense to me at all:

layering a ton of images into one massive image that spins up into a multi-functional container is not efficient. The great thing about containers is that it tricks the computer into using it's un-utilized processing power to run many more containers at the same time than it would if they were regular pieces of software, thus getting more 'bang for your buck' out of the host machine (so-to-speak). Having your entire app's functionality in one container wastes all that precious processing power Docker could have been squeezing out of the host if you otherwise separated your app into discrete, containerized microservices.

How does splitting up different services into different containers make better use of processing power than if you packed the services into one container? Are there restrictions on multithreading/processing in docker? If so, worth explaining.