Dear Eebie,
You have a restrictive reading of the guidelines, which by the way are guidelines, not rules.
The guidelines do not say anything about relations, which are logical by definition.
If we were to follow your restrictive view, one would have to delete all maritime lines because they could not be verified on the ground. Bus lines should be removed as well because, unlike railways, there are no physical markings on the roads, a part bus stops. International bus lines such as Eurolines could not be mapped. Same would go for airlines.
Moreover, as regards the walking route I created as relation, it fully complies with the "Verifiability" criteria since the route is described in a real book which can be bought or read by anyone.
Finally, you should read again the introduction of the guidelines that says "Nobody is forced to obey them, nor will OSM ever force any of its mappers to do anything. There might be cases where these guidelines don't apply, or even contradict each other." Not to mention the "Any tags you like principle" (osm.wiki/Any_tags_you_like) which reads "Remember that OpenStreetMap does not have any content restrictions on tags that can be assigned to nodes, ways or areas. You can use any tags you like, but please document them here on the OpenStreetMap wiki, even if self explanatory." ...
Thank you for your understanding.
Best,
gerdami

Dear Eebie,
Thank you for contacting me.
I just updated the link to the SGR publication (thanks to Google).
The "randonnées en boucle" (roundtrip routes) actually reuse parts of neighbouring GRs routes, thus share parts of their markings.
Hope it helps.
Regards,
Gerdami