There’s truth to that yes. I don’t know about synthetics though, especially for this higher range. The problem I see for many students is they don’t cover the necessary basics needed for gaining control of the altissimo range. So, if you don’t master the harmonic overtone series but think you can hit the altissomo notes because you got a stiffer reed then it won’t help. For someone who is already playing that range quite well, then yes, it is easier with a slightly stiffer reed, just as a thin or worn out reed won’t give as much control.
Hope that makes sense!

Johnny