I think nowadays with the myriad of tools available there's no excuse for doing an app for just one platform, I struggle with doing exercise so I opened the link excited to be completely disappointed by the fact that it's only for iOS.
OP wrote an app to solve their own problem and made it available for free for others to use if they find it helpful. It should shock no one and certainly should not trigger any judgement or attacks from anyone that they chose to write the app for the platform which they themselves use.
Honestly, your initial comment comes off as whiny and entitled: you lost maybe 30 seconds of your time clicking through to something that wasn't relevant to you, and there's "no excuse" for leaving a drive-by comment attacking the author for their choice of tech stack for a side project just because of that tiny inconvenience.
If OP made the app cross-compatible it wouldn’t look as nice on iOS, or he’d have to essentially make two separate frontends. You could argue for either option but it’s not “no excuse”.
If there's one thing I've learned from a few decades on the internet it's that the only way to avoid entitled whining is to never, ever release anything, especially not for free.