Judging from the unkempt hair and torn Starfleet uniform, he may be more than an illusion.
The major thing to note here is that Culber appears to be in the Mycellial Network, and may very well be alive within it.
That was something that was speculated in Season 1 when Stamets was stuck in there, but it wasn’t entirely clear whether Culber was actually there or some hallucination that helped his partner find his way back into the real world.