Absolutely true, but going down in gauge can take care of that and then some.
The lightest set of Ernie Ball Slinky Flatwounds (aka "cobalt flats") is 40-60-75-95, and its tensions at 35" scale - i.e. 41.77, 46.41, 34.52, 34.44 lbs - are lower, and in one case a lot lower, than tensions of a 45-65-80-100 set of Stringjoy roundwounds at standard, 34" scale (44.9, 50.1, 45 and 37.8 lbs).
Even assuming the flats, being stiffer, will feel tighter than a set of rounds with the same gauges on the same scale, I doubt their perceived extra tightness will outmatch 4–10 pounds' worth of extra tension proper from a larger-gauged roundwound set (at a shorter scale, to boot: add 2.5 pounds per string to the Stringjoys if we want to compare them to the cobalts on the same, 35" scale bass).
On the other hand, not everyone likes the feel of thin strings under the fingers: for those players, going down in gauge wouldn't be an option.