Not quite. What kind of writers are they? would be Ők milyen írók?
The difference lies in whether milyen(ek) is a phrase on its own or part of a larger phrase. Similarly, in English, in what kind of writers, what kind of is a part of the larger noun phrase what kind of writers. To express this in Hungarian, milyen is also part of the noun phrase: milyen írók.
In what is x like in English, in contrast, what is a phrase on its own. The same is true in Hungarian, with milyenek, only that the Hungarian expression has to agree in number with whatever x stands for, thus: Milyen az író? ‘What is the writer like?’ and Milyenek az írók? ‘What are the writers like?’