Yeah, it’s not a rumor but a fact stated by ANet. More than half of the ppl in ANet are working on the next expansion as we speak here.
Actually according to Mike in the april ama (https://www.reddit.com/r/Guildwars2/comments/48zlyd/im_mike_obrien_here_with_gw2_dev_team_ama/) 120 devs are working on live, 70 on expansion2 and 30 are working on core systems. If ANET hasn’t released new numbers I’ve missed only a third of the company is working on xpac2.