There is a difference between total understanding and empathy. As with any marginalized experience, being trans will never be understood by those who have not lived it. However, it is still entirely possible to attempt to empathize and to care, while still acknowledging that cis people are not an expert in the trans experience.
Being cisqueer, frankly, gives near-to-no insight into the trans experience except by proximity of our communities. It is long past time that cisqueer people acknowledge that while they can and should ally themselves with us, they cannot speak for us, nor can they understand our lived experience fully.