Nobody else interprets the tag in that way. Note how the cyclOSM rendering treats it differently. Cycle routing algorithms include "dismount" sections - so it is important not to use this tag for places where cycling is prohibited. 

"dismount" is used for stretches of legally designated cycleway are dangerous to ride (eg the Lancaster Canal tow path by the Water Witch). Not places where it is illegal such as pavements or motorways. 

The most common use of the cyclist dismount sign  in the UK is to mark the end of a cycleway (as is the case here) where cyclists must rejoin the carriageway. 

So the more relevant question is why a mapper who claims that "dismount" and "no" mean exactly the same thing would be at all interested in which tag was used.

And since the restriction on pavements is identical to that on motorways (in both cases riding a bike is illegal - in both cases a legal user of the way can transport a bicycle so long as they don't ride it), I am anticipating you "correcting" the cycle access tags on the M6 in the near future.
 

