That's an oversimplification.
- iOS does multitask. How does one play music in the background while surfing, how does the os keeps alarms and schedules while you run other apps or play games? Don't confuse multitask by having apps in memory. Android put apps to sleep as well. Try using an Android phone with 2GB of RAM, and you'll find you can barely keep an app open, while iOS do better with the same amount of RAM. If Android needs 6GB of RAM to "multitask," that doesn't say much. Windows can multitask better with less RAM than Android.
- As an architecture, Android already at a disadvantage. Just the way it load apps and draw them are already "laggy" compared to the likes of iOS and even Windows Phone. Go back a few years where low end Windows Phone runs smoother than even a mid-range Android phone. It's the architecture itself. There's a reason why Google themselves is trying new options, eg. Fuschia OS, Android Go, etc.
- Luckily for Android, Moore's law is helping the OS perform better with leaps of performance improvements on every new snapdragon iteration.