So is this a slight mistake on Herge's part?
Yes,but a very minor one. He only had one reference book for background material, Joseph Douillet's 1928 Moscou Sans Voiles: Neuf ans de travail au pays des Soviets
, which is a fairly dull and stodgy text; Hergé takes several incidents more or less straight from the book, but he doesn't appear to have read it very thoroughly, nor completely, probably through lack of time as much as anything else.
As the book covers a period of years, but not in chronological order (it covers things thematically, rather than as a straight history) there are mentions of both the Cheka (which my 1930 copy in English translation refers to as the "Tcheka") and OGPU.
The waters are muddied further by the fact that the OGPU didn't follow directly from the Cheka - strictly speaking, the lineage is Cheka (1917–1922), GPU (1922-23) and then OGPU (1923–1934).
It would be easy to look through and pick out incidents, or ideas, and to note that the OGPU or Cheka (or GPU) were involved, but not realize that one organization followed the other historically; Drouillet contributes to this himself, as when he writes of the GPU in the present tense in 1928 - "The facts related above will suffice to show the reader how GPU punishes ordinary citizens of the U.S.S.R...." [p.31 of the 1930 Pilot Press edition I have].
Whether or not the actual staff changed, and one organization inherited the staff of the next I don't know; that's beyond any expertise I have, but I'd guess that it's more than likely.
You could also consider that it's possible that it's the character
who is being depicted as unaware of the change, or just not really bothered by it; it's not uncommon for people to go on calling a group, organization, or facility by a name, long after that name has changed (people here in the U.K. often talk about "British Rail" when discussing rail travel, even though that was replaced by various companies when the rail service was privatized years ago).