If you object to that particular implementation, the point is there's a usage API in case you'd like to write your own app.
"Whenever you want to focus, plant a tree.
The tree will grow in the following time.
The tree will be killed if you leave this app."
So probably, yeah. I assume there is some point at which the tree won't be killed since it goes on to talk about keeping a forest of trees representing stretches of time not using your phone to do anything other than run this app - hitting up its app store page describes it as a pomodoro timer, so about 25min gets you a tree.
My inner paranoid says "wow, what a great way to get people to help you mine cryptocurrency" but I will choose to assume that they're happy with the money they're making by selling it for two bucks, plus the occasional "sunshine elixir" IAP.