The startup's engineers had built their entire system assuming that user demand would be highest during weekday business hours, based on…

The second sentence reveals that actual usage patterns diverged sharply from the assumption stated in the first sentence, and in fact signals exactly that kind of correction, revealing what turned out to be true in contrast to what was assumed. For this reason would incorrectly suggest the actual usage pattern was a consequence of the original assumption, reversing the logic. Specifically would frame the second sentence as a more detailed version of the first's claim, but the two sentences actually conflict instead of one elaborating the other. Similarly would suggest the two sentences align, when the entire point is that they don't.