In his translation of the short piece "Father's Butterflies" by his father, Vladimir Nabokov, Dmitri Nabokov has a neat little reference to "the terrible turtles who direct learned journals."
As the quote says, the piece "Father's Butteflies" should have the quote. In the link Peter Shor kindly provided, which is to that work, in part 4, about 2/3rds of the way down the page (ctrl-F is particularly useful to find this):
I like to think he was not mistaken here, and that, in time, men will appear who are more alert than Murchison, more educated than I, more talented and lively than the terrible turtles who direct learned journals, and that the elaboration of my father's thoughts, jotted in the hasty hand of a testament in the night preceding a dubious departure, when holster, gloves, and compass intrude momentarily on the sedentary life of the desk, and pursued here in a haze of filial love, piety, inspiration, and mental helplessness, will create a worthy monument to him, visible from every corner of natural science.
(All bolding my own. Thanks to Peter Shor for finding the link.)
Meaning and context of essay
For the meaning, a wee bit of context. The essay is about butterflies and a classification for them that Nabokov proposed along with some other plans. While his writing is apparently very difficult to understand, Dmitri is suggesting that Vladimir was not mistaken that
a maximally accurate exposition of the principles of such a study would still allow minds that at last understood them a chance to consummate the plan outlined by the author.
In other words, Dmitri is suggesting that the plans he outlined could be done by someone who is "more educated than [himself]", "more alert than Murchison" (Murchison was the author of a 300-page explanation of Count Godunov's thirty-page treatise, and is referenced earlier in the treatise as someone "whose lepidopterological knowledge is very limited"), and finally "more talented and lively than the terrible turtles who direct learned journals".
We thus reach the meat of the matter. As the quote you quoted in your question points out
The Russian connotation is the nightmarish iniquity of mediocre thinkers holding considerable power over one's output and career path
which conveys the main point. I'll just add to it slightly by saying that this makes sense as it references the talents of the "turtles" implying that they are not very talented, and also that they are "terrible", one connotation of which can mean "having great, awful power". Since the quote you referenced in your question talks about the word "terrible" not fully conveying the awfulness in the original Russian, this meaning makes the most sense to me.
Finally, the turtles part - note this connects to the fact that Dmitri points out that the he is looking for men who are "more [...] lively" than the turtles. A turtle is sluggish, by definition has a lack of liveliness. This connects with the lack of talent (their brain is "sluggish") and also just their ineptitude - incapable people can be referred to as "slow".
After the paragraph in which the turtles appear, Dmitri laments the unfinished-ness of a life's work, saying
The bitterness of interrupted life is nothing compared to the bitterness of interrupted work: the probability that the former may continue beyond the grave seems infinite when compared to the inexorable incompletion of the latter.