6-A key Cui Heng can't interact with him but his Return To Void key can and as I've previously stated, he can become that key.
Also to elaborate on becoming that key if you missed it on the profile, his Return To Void key was fighting a 2-B, possibly 2-A enemy and that enemy attacked through time to kill Cui Heng's past 6-A self and despite such an immeasurable speed attack,
Cui Heng reacted before he was hit and instantly became his Return to Void key.
There are countless versions of him in his time, in his past and in his future, all of which are like a hivemind and can perform the above feat.
Diablo needs layered 3D regen negation to do that to Cui Heng or 4D or above for his 6-A key and that would be temporary considering he can't die unless all his countless versions are also dead on the timeline (check his immortality type 8 in this key).
It is a pretty large verse so I'll wait on many more people to share the same view.
Cui Heng has type 2 info in his Return To Void Key.
Cui Heng's second key has immortality negation which covers all his immortalities in his first key. Also immortality type 9 negation is the ability to simultaneously attack the avatar and the true self which also negates Diablo's immortality type 8.
Lastly type 5 is covered by his Soul Formation key that can induce death onto the essence/spirit of a person which is deathless, indestructible, timeless, cannot be erased, cannot be restrained, etc.