Well, the linked calculator seems bogus (at least for me here in Germany). It assumes that the income is AFTER taxes. Let put some numbers in:

Given I am a programmer in Germany And I earn 4000€ (above avg.) before taxes [1]

That means I will have around 29000€ after taxes and other social charges [2]

Now if I only spend the bare minimum (9000€) [3], I can save 20000€ per year.

Now I am able to retire after 9.5 Years according to calculator. This seems right, as if I still want to live off 9000€, but guess what: The living costs (even if I do stay single, live in the same small flat and never do any vacation) rise with the years.

Now you could argue, that a saving rate of 68% might be too much, but even if you decrease the saving to 10000€ per year, you are working around 25.2 years which is plenty of time to burn out...

[1]: https://www.gehaltsvergleich.com/gehalt/Programmierer-Progra...

[2]: https://imgur.com/a/IArku

[3]: https://de.wikipedia.org/wiki/Grundfreibetrag_(Deutschland)