There are operational tailless aircraft such as B-2 spirit, X-45. It seems yaw stability/control at slow speed is achieved aerodynamically, using either split elevons (B-2) or adjacent elevons deflected in opposite directions (X-45) Combined with negative wing tip twist angle, both solutions increase drag / relative swept angle, and by the way, yaw stability. Also note both examples have a very small vertical surface ahead of CoG.
On the other hand, looking at this picture of a F/A-XX Boeing concept, it seems vertical surface ahead of center of gravity is quite significant, and forward swept trailing edge is in contradiction with aerodynamic stabilisation using split elevons.
One might think this isn't such a big issue since this concept might use permanent vector thrust adjustments on yaw axis.
Question is : Does it fly with no yaw thrust vectoring?
Edit: One other issue affecting yaw stability is forward swept elevons. Their "split brake" efficiency decreases while sideslip angle increases :