Ah, I knew I would see this hot take here! I responded elsewhere, but in short form, this is very likely far more driven by Tumblr's new corporate parents wanting to make the site friendlier to advertisers.
Isn't Reddit allowed on iOS?
Reddit has been removed from the app store more than once.
A guess would be that no, they weren't going back on the store without adjusting their rating to R.
And obviously showing that they can keep child porn off their service.
Citing Kotaku "In an email a spokesperson from Tumblr clarified that the removal of adult content was not related to the app’s removal from the iOS store."