Reach
The number of distinct people who saw an ad at least once, as opposed to the number of times it was served.
What makes it wrong.
The mechanism, not the warning.
Reach cannot be added across date ranges. Two weeks of a hundred thousand reach each is not two hundred thousand people, because the overlap is unknown, and pulling the two weeks as one range gives a different and correct number.
In detail.
Reach is deduplicated and impressions are not, which is why impressions divided by reach gives frequency. It is the denominator that matters for saturation questions: whether an audience has been exhausted, and whether spending more will find new people or the same ones again.
Deduplication only holds inside one platform. The same person reached on two platforms is two reached people in the sum.