To put it into numbers: they have approximately 8 million accounts [0]. A license costs 4€ now, but it used to be cheaper (2€), so using the older number and subtracting fees and taxes, gives us 11 million € of revenue in the past half decade. That's not a giant number, but enough to support development of an app like this.

[0]: https://de.statista.com/statistik/daten/studie/445619/umfrag...