This question appears to be off-topic because it is about Latin, not English. Educated writers are free to add any non-English expression to their prose, but we are not required to weigh in on such, nor is it appropriate for us to do so.

@Robusto: I disagree... It is about Latin, and English. This phrase appears to be used in English just as "sina qua non" or "QED" are. Yes, it's Latin, as are many other English phrases. If you assert that the phrase is not well-known enough in English communication be counted as on-topic, please cite your data to support that assertion. I'll add mine to the question.

@Barmar: I thought that was odd too. Did it only recently start being used in English? Not even datum sed non concessum is in there.

@Barmar: I would if they involved an analysis based on Latin grammar. Also, the ones you mention are way more frequently encountered than datum non concessum.

@Robusto: I included a cursory analysis of the Latin meaning of the phrase, as relevant background information, but my question is not about the Latin meaning. It's about the English meaning. Regarding your second sentence, can you be explicit about how objective measures of occurrence of datum (sed) non concessum establish that it is not an English phrase?

@choster: Thanks for your comment (answer). Do you have any references?

@LarsH: If you don't think difference of degree can be relevant, try an electric shock from a 9v battery and one from a bolt of lightning.

@Robusto: A valid point (datum et concessum). Yet the comparison doesn't imply that a 9v battery is inadequate to power a smoke detector. Nor does "X is used way less than Y," by itself, establish that X is not English.

@LarsH: I thought we had already established that Latin is not English.

@Robusto Barmar and I gave examples of other Latin phrases that are clearly English phrases. So if you mean we've established that no Latin phrases are English, then no, we've established that some Latin phrases are English.

@LarsH I mean only that something so abstruse can never really be English. Currency is important in this case.

And "some Latin phrases are English" is not "all Latin phrases are English."

Or, rather, to say that some Latin phrases are English is not to say that all Latin phrases may be considered to be English.

In any case, mine is but a single vote. So far I think that question remains open.

@Robusto I agree with all of that with the exception of "something so abstruse can never really be English." It remains to be demonstrated that datum (sed) non concessum is that abstruse. It's possible, but nobody has presented data in favor of that conclusion.