I am sure that Tutanota does not use any custom encryption algorithm. It is clearly stated in the FAQ that they use RSA (with PFS) and AES to encrypt emails exchanged between Tutanota users. https://tutanota.com/encryption
These are only primitive algorithms, the actual implementation is custom and specific to Tutanota, which mean it will only work with Tutanota as nothing else will implement it.
There is no way to do key distribution outside of Tutanota's service.
It's probably also media's fault for this. They only publish when a bad person does a bad thing on the internet with it, not all the millions of users who don't do bad things. That would be boring.