Boo, uncool. Shouldn't have announced it at all if it were that unfeasible.
I realize that letting people outside Tumblr read Tumblr posts means losing ad revenue on new users, but keeping Tumblrites on Tumblr and allowing them to bring in Mastodon/Lemmy/pixelfed posts would keep the existing users more glued to the platform (more ad revenue). I guess they're gunning for new users primarily.