The footway from Lothianburn to Carlops seems to consist of a number of segments, all of which are tagged as "bicycle = designated". If nothing else that's contrary to OSM's own guidelines which state:
bicycle=designated is applied where cycling is not only legal, but object is explicitly designated for use by cyclists.
...
bicycle=yes is applicable where it is necessary to mark that cycling is legal, but there is no explicit designation for use by cyclists.
I've been up and down various bits of the A702 recently and I've seen no shared use signs on the footway anywhere (doesn't mean that there aren't any, but if there are any I've not seen them). So the "designated" tag seems inappropriate. And AFAICS the only way that the "yes" tag could be justified would be
1) Under the terms of the guidance from the Minister for Cycling in 1999, as reiterated in 2014, and as acknowledged by the NPCC on its web site, on the use of discretion by CPOs in issuing fixed penalty notices for pavement cycling;
2) A misunderstanding of the open access regulations in the Land Reform (Scotland) Act, as debated here some time ago.
In either case, though, I'd expect the result to be that the majority of pavements would be tagged as "bicycle = yes" on OSM, either in Scotland or UK-wide.
Alternatively, I suppose it might be a stealth attempt to get that footway accepted as a de facto shared use path, with the aim of persuading Midlothian council eventually to give in and designate it so? Given how little it seems to be used by pedestrians I'm not sure it would really need any specific infrastructure changes to accommodate considerate cycling, beyond the appropriate signage (though the applicable regulations might say otherwise).
In the absence of authoritative information to the contrary, I'm afraid I'd be inclined to put it down to wishful thinking. Which is not to say that I don't think it would be a stonkingly good idea on that stretch of road - and I certainly sympathise with the folks I do see riding on that footway preferring to keep out of the way of some of the idiots who drive along that road.