I think that what Jeet Heer had in mind is the following:
Proustian, adj. 2. Of, relating to, or characteristic of Proust, his writings, or his style, particularly with reference to the recovery of the lost past and the stimulation of unconscious memory.
Oxford English Dictionary
One of the features of Proust’s Remembrance Of Things Past is a first-person narrator who is interested not only in the things that happened, but also in the process of remembering those things. The episode of the madeleine from Swann’s Way is well known:
Many years had elapsed during which nothing of Combray, save what was comprised in the theatre and the drama of my going to bed there, had any existence for me, when one day in winter, as I came home, my mother, seeing that I was cold, offered me some tea, a thing I did not ordinarily take. I declined at first, and then, for no particular reason, changed my mind. She sent out for one of those short, plump little cakes called ‘petites madeleines,’ which look as though they had been moulded in the fluted scallop of a pilgrim’s shell. And soon, mechanically, weary after a dull day with the prospect of a depressing morrow, I raised to my lips a spoonful of the tea in which I had soaked a morsel of the cake. […]
And suddenly the memory returns. The taste was that of the little crumb of madeleine which on Sunday mornings at Combray (because on those mornings I did not go out before church-time), when I went to say good day to her in her bedroom, my aunt Léonie used to give me, dipping it first in her own cup of real or of lime-flower tea. The sight of the little madeleine had recalled nothing to my mind before I tasted it; perhaps because I had so often seen such things in the interval, without tasting them, on the trays in pastry-cooks’ windows, that their image had dissociated itself from those Combray days to take its place among others more recent; perhaps because of those memories, so long abandoned and put out of mind, nothing now survived, everything was scattered; the forms of things, including that of the little scallop-shell of pastry, so richly sensual under its severe, religious folds, were either obliterated or had been so long dormant as to have lost the power of expansion which would have allowed them to resume their place in my consciousness. But when from a long-distant past nothing subsists, after the people are dead, after the things are broken and scattered, still, alone, more fragile, but with more vitality, more unsubstantial, more persistent, more faithful, the smell and taste of things remain poised a long time, like souls, ready to remind us, waiting and hoping for their moment, amid the ruins of all the rest; and bear unfaltering, in the tiny and almost impalpable drop of their essence, the vast structure of recollection.
Marcel Proust (1913). Swann’s Way. Translated by C. K. Scott Moncrieff.
Gene Wolfe also used first-person narrators who were interested the nature of memory and the process of remembering. In The Book of the New Sun the narrator Severian often makes remarks like these:
Just as all that appear imperishable tends towards its own destruction, those moments that at the time seem the most fleeting recreate themselves—not only in my memory (which in the final accounting loses nothing) but in the throbbing of my heart and the pricking of my hair, making themselves new just as our Commonwealth reconstitutes itself each motning in the shrill tones of its own clarions. […]
It is my nature, my joy and my curse, to forget nothing. Every rattling chain and whistling wind, every sight, smell, and taste, remains changeless in my mind, and though I know it is not so with everyone, I cannot imagine what it can mean to be otherwise, as if one had slept when in fact an experience is merely remote. […]
When I think back on that time, it is that moment I recall first; to remember more, I must work forward or backward from that. In memory it seems to me I stand always so, in gray shirt and ragged trousers, with the blade poised above my head. […]
As I have said, I remember everything; but often I can find a fact, face, or feeling only after a long search.
Gene Wolfe (1980). The Shadow of the Torturer. New York: Timescape.
Wolfe’s novel Soldier of the Mist (1986) is also concerned with memory, but in this novel, the first-person narrator has a head wound that has makes it impossible for him to form new memories, so that as each day begins he has to figure out who he is by re-reading his own written narrative.
Another similarity between the prose styles of the two writers is the narrators’ penchant for gnomic aphorisms. Here’s an example from Swann’s Way:
Perhaps the immobility of the things that surround us is forced upon them by our conviction that they are themselves, and not anything else, and by the immobility of our conceptions of them.
And here’s one from The Shadow of the Torturer:
A crowd is not the sum of the individuals who compose it. Rather it is a species of animal, without language or real consciousness, born when they gather, dying when they depart.