This is a myth IMO. Where there are users and the means for making payments, there will be enough buyers for apps, for as long as your supply meets their demand.
Unfortunately for app developers, the gold rush for the App Store is long over and these days it is getting harder and harder to get a return on selling your apps. This is not surprising actually, given that most apps are shit. Seriously, in case you haven't noticed, besides a handful of apps that are actually useful and a handful of games that are somewhat fun, all other are shit.
I still remember when VLC was banned from the iTunes App Store, supposedly due to some patents stuff, leaving in place dozens of click-baits shamelessly using VLC's name. I actually bought a video player app in the hope that I'll find a suitable replacement, but you guessed it, it was shit. So then I returned it, after searching on Google for how.
As both an Android and iOS user spending money on both, in my case especially for games due to my son, you begin noticing that Android's free apps are better, Android having an actual open-source ecosystem. Of course, things aren't perfect and for iOS at least I found and bought a decent RSS reader, but then I ended up paying for Newsblur and using it on Android which also counts.
The point I'm trying to make is that: (1) iOS will probably not make your app sell better if your app is shit and if your app is any good, it will sell well on Android too and you'd be a fool to not release it on both and (2) if people buy more apps on iOS, it doesn't necessarily reflect well on iOS.