The quote doesn’t say explicitly, so just to make sure we’re on the same page: I take from this that yes, when more looms were tended, each loom required less attention. Do you agree?
The significant thing is that it was not symmetrical, ie they didn’t replace one machine with two machines that each took half as much attention. Working 8 took 80% of the time, so it looks like the 1900 machines each only took 10%, compared to the earlier single machine which took 30%. This suggests to me that the new machines took ~0.33 the attention the earlier ones did. So the improved machines lead to workers “only” working about three times as hard overall.