Upvote:2
On the page you are linking to, you can find in the section 'Fragen und Antworten ...' (questions and answers) a more detailed description about what they mean with 'mostly'.
International tickets can be used flexibly until 2020-10-31 with the following restrictions:
Since no restrictions are mentioned here for domestic connections, be it in the Netherlands or anywhere else, I would very much assume that you can use your ticket as you want to.