I think you’re underestimating Apple’s chip performance. Going by https://browser.geekbench.com/ the best performing Android devices(Samsung’s special thing for single-core, Snapdragon 845 in a OnePlus for multi-core) appear around even with an A10 for single-core and an A10X with multi-core. If this/last year’s Android chips were enough to make Dex viable, you’d expect last generation’s iPad Pro and the iPhone 7 to be capable of running Apple’s solution.

The Snapdragon 835 is not the enabler here. A mid-range 5-year-old phone has 4 cores at 2+ GHz, more powerful than a 10-year-old computer. Even back then this was powerful enough to run plenty of desktop workloads. The thing that's enabling usage of phones as desktops is USB Type-C which is fast enough and versatile enough to carry video output and a USB singal while powering the phone.