Sounds like simply shitty programming if they need a 15 minutes delay in order to give users the possibility to edit their posts .. editing posts is possible in lots of fediverse software without such hacks lol
But it's cooperative, as long there is 1 instance that doesn't respect delete requests, they can't provide the ability to delete a mistakenly posted picture that actually works.
15 minutes is excessive though. 3 minutes should be enough.