This is not true - Niantic do not have this rule if you read the Criteria Clarification post for Trails and Markers at Trails & Markers
" Although these are mass produced, they are official and at a decision point. The object and its function are unique. The supporting photo and website of the route will help verify this rural location."
The UK generic rights-of-way markers are not part of named trails and do not have a logo, yet have been described as definitely eligible by Niantic.
They are generally accepted by reviewers for Pokemon Go, as long as they are not on streets pointing at paths into fields, even if they are not at changes in direction.