As you know, looking for strict logic in OSM tags is an unfruitful exercise. For better or for worse, leisure is the top-level key for all kinds of sport grounds, and leisure=track has been used for similar structures, Wiki even addresses the issue at hand, bold mine:
A dedicated track for running, cycling and other non-motorised racing such as horses, greyhounds, typically not a part of the normal network of ways and paths.
This is not a replacement for the normal highway=* values for ways that are a part of the general network of ways but which are used for non-motorised sports. For instance: a path that in a forest with a mountainbiking route on it, is still a highway=path.
So, if used primarily for mtb downhill recreation and competition, it makes perfect sense to use leisure=track; if it’s just a mixed-use path I agree it should be a highway=path.
Conceptually, single-use downhill tracks resemble ski pistes, but their current tagging scheme is unlike anything we have: the top-level tag for a downhill ski piste is piste:type… 