GR translation: All his vacuums, his labyrinths, had been the other side of this

[Mike Jing]

V432.32-433.1, P439.37-440.6   The odors of shit, death, sweat, sickness,
mildew, piss, the breathing of Dora, wrapped him as he crept in staring at
the naked corpses being carried out now that America was so close, to be
stacked in front of the crematoriums, the men’s penises hanging, their toes
clustering white and round as pearls . . . each face so perfect, so
individual, the lips stretched back into death-grins, a whole silent
audience caught at the punch line of the joke . . . and the living, stacked
ten to a straw mattress, the weakly crying, coughing, losers. . . . All his
vacuums, his labyrinths, had been the other side of this. While he lived,
and drew marks on paper, this invisible kingdom had kept on, in the
darkness outside . . . all this time. . . .

What does "had been the other side of this" mean here?