In the mean time I've checked the terms of service of the Rother app. They say
> The rights to all content provided in tours and guides are held by Bergverlag Rother GmbH
> It is expressly forbidden to the user to export the tour other than with the export function provided to the user, to pass it on or to publish it elsewhere. 
https://rother.app/tos

This is hardly surprising because they charge for it. To redistribute the GPX tracks threatens their business model.

I'm worried that this will get OSM into trouble.

Why don't we ask the authors of Walkhighlands if they will make the GPX tracks of their walks available under an open license. They run on donations and advertising and don't depend on charging users money for the tracks. If they say yes, then we can put them somewhere permanent, maybe the Internet Archive, and you can link to there from Wikivoyage?