So you jump to "new elf" instead of "new night elf content"? Your own words betray how non-confirmed your conclusion was. Out of the millions of players is a 99.9% certainty that no one figured out "Nightborne" as a new race based on the Mystic Runesaber mount. It didn't mention a new race. It didn't mention new elves. They didn't dump tons of lore onto the model as well. All mounts have brief descriptions attached to them. That is neither "tons" or "deep". Stop stretching and reaching for everything but the kitchen sink.
RMT Store items can hint at a future expansion but there is no rhyme or reason to the way they do. Some do. Some don't. At most we can speculate that it might be involved but there is no way to know for sure until after information gets revealed. It has always been that case because Blizzard has never used store items to spoil the expansion reveal.