After pondering that for some more, I think the correct openstreetmap tagging from this statement would be.
highway=road + bicylce=yes for bike paths and
highway=path + bicylce=yes for mtb trails.
Such nonsense though reminds me of what a German map producer wrote in another topic:
Use highway=path for the shared use municipally constructed and maintained infrastructure, a.k.a. bike paths; and use highway=trail for all those unimproved backwoods hiking trails or purpose built MTB trails, where consumers are urged to look for tags like mtb:scale or sac_scale, in case they are even interested in consuming them. From a non-native English speaking point of view that sounds reasonable.