What you are referring to is the past perfect. The term pluperfect is rather antiquated, I would say.
The "past perfect" is not a tense. It's an aspect of the past, or a way in which we can talk about the past in a specifc way.
You did indeed say that the term pluperfect is antiquated, and that it is now called the past perfect, and that it is not a tense. I think you had forgotten, since you wrote above to our OP that you hadn't made that statement, after he or she went to the trouble of verifying it.
In any case, a linguistic debate is good fun, isn't it?
What I really mean by '"in that case" the OP needs to use the pluperfect' is this: when the state of affairs in question includes three times (present, past 1, and past 2) and when one of them takes place just moments before the present (the 'uh-oh, I thought' moment) and the other past moment is well before that (last week-end's attempted e-mail reply) then I don't think it's anything more than a colloquial eliding of the facts to use the phrase 'I thought I replied,' because it simplifies the time line to the point of amalgamating all past moments into one. In such cases, I think it's an imprecise, almost 'lazy' approximation to write it in that way. "I thought I had replied," on the other hand, correctly accounts for the temporal dimensions of the state of affairs as it actually unfolded. The "uh-oh" moment is accurately portrayed as a recollection of a
previous attempt to reply, that was significantly
before that "uh-oh" moment. It also shows that the speaker was for some time convinced that the response had been previously sent.
So, if you compare an approximation that is unlikely to be misunderstood only because logically you can't think you replied before you try to do so, on the one hand, and on the other hand we have an expression that explicitly accounts for the facts in a complete and unambiguous way, I think we can observe a differing value judgment regarding the relative correctness of the two, in terms of the norms of written English, which are themselves more conservative, more universal and more international than any one vernacular.
I just felt that given a learner who had been reading a textbook explaining the use of the pluperfect, we ought then to have used the textbook's standard of written English when discussing normative correctness, rather than troubling the student with opinions about whether more recent or more local standards should now replace those of the textbook.
To summarize, "I thought I replied" I am still convinced is an oral approximation rather than an equally well constructed alternative to "I thought I had replied" if we are talking about the sequence of events I described above.
I am sorry if my love for such linguistic debates has detracted from anyone's pleasure at using the forum.