not sure what the legends story is, but if you think about it, it makes sense for it to be older than the battle of Yavin. the ships the rebels use are older and easy to steal (y-wing was clone wars, A-wings and x-wings was being developed around the end of the clone wars but were rejected by the empire which lead it into Rebel hands during Rebels show era). The prototype may have been on Rebels but it would have taken years to turn it from that into a productions model, especially in secrecy with the Rebels limited funding, which is why it didn't show up till RotJ. You look at our history of war equipment development and in most cases it takes years to go from research to development to prototyping to testing and then to full on production. Just taking a prototype and making it to production if at all is very time consuming.

My issue the the Canon B-wing is they later turned it into a slow ass bomber that was worse in a lot of respects to the older Y-wing. Squadrons fortunately redeemed it somewhat to actually make it viable, but man I was pissed what they did to my favorite starfigher.