ARM is behind Intel on single core execution speed but that isn't entirely comparable because the instruction sets aren't 1:1. It isn't a limitation of the chip design and running multiple threads on different cores is a feature of the XNU kernel. Apple have shipped a multithreaded kernel under the hood for iOS since the beginning and whilst you might only interact with a single UI application, there are numbers of processes running behind the scenes.
The demo during the keynote featured the same chipset as is in the iPad Pro, an A12Z. It has more memory (16GB) than the iPad Pro does (6GB) and there is part of the problem. Running multiple applications requires RAM for each of them or they need to swap when you run out of RAM. In general you want to avoid swapping so when there is memory pressure, iOS will terminate applications that were running in the background to make sure memory is available. MacOS also has a similar process as well though less aggressive than on iOS. On iOS once you run out of memory, your app is killed.
That's not an ARM limitation though, that's a memory limitation and a design trade off.
