Tumblr is perfectly fine on the web, but they want people to download the app so they can use push notifications to juice engagement.
As of Aug 2021, PWA support basically didn't exist except for some parts of the manifest file. No background tasks, none of the major APIs were implemented, and you got nothing but an app tile on your homescreen.. even navigating back and forth from the PWA to another app caused the PWA to lose state. Data storage was basically a nightmare.
There are STILL basic fundamental things missing.
When the tumblr saga was going on, I still don't think you could even read PWA manifests or add them to your homescreen!
None of those things are necessary to create a very full featured Tumblr web application. E.g. twitter works quite well on mobile web.
Tumblr wants people to download the app for engagement purposes.
2. Safari had and still has some pretty gnarly bugs that require special workarounds.
3. You could not put a link to a website on the homescreen of iOS