I don't think this is true. Could you source this? I've had several small outdoor electric hot tubs, and all of them have had dedicated heaters. The couple times I've had problems with the heater, the symptoms involved the pump running constantly and the tub never reaching temp. I'm sure there is some friction heating, but I don't think it's anywhere near enough to reach standard temperature.
Apparently you're correct, and I was misinformed. You could implement it this way—like how Tesla's newer designs can run the electric motor in a less-efficient mode to provide heat for the battery pack—but the designs I investigated did have separate heaters as part of the circulation system. (For some reason the specs for these were not listed in the product summary, but rather only in the manual.)
S̶p̶e̶c̶i̶f̶i̶c̶a̶l̶l̶y̶,̶ ̶f̶o̶r̶ ̶t̶h̶o̶s̶e̶ ̶w̶h̶o̶ ̶w̶e̶r̶e̶n̶'̶t̶ ̶a̶w̶a̶r̶e̶,̶ ̶s̶m̶a̶l̶l̶ ̶h̶o̶t̶ ̶t̶u̶b̶s̶ ̶a̶r̶e̶ ̶t̶y̶p̶i̶c̶a̶l̶l̶y̶ ̶h̶e̶a̶t̶e̶d̶ ̶t̶h̶r̶o̶u̶g̶h̶ ̶f̶r̶i̶c̶t̶i̶o̶n̶ ̶a̶n̶d̶ ̶w̶a̶s̶t̶e̶ ̶h̶e̶a̶t̶ ̶f̶r̶o̶m̶ ̶t̶h̶e̶ ̶c̶i̶r̶c̶u̶l̶a̶t̶i̶o̶n̶ ̶p̶u̶m̶p̶s̶ ̶a̶n̶d̶ ̶d̶o̶n̶'̶t̶ ̶h̶a̶v̶e̶ ̶d̶e̶d̶i̶c̶a̶t̶e̶d̶ ̶w̶a̶t̶e̶r̶ ̶h̶e̶a̶t̶e̶r̶s̶.̶ ̶Y̶o̶u̶ ̶c̶o̶u̶l̶d̶ ̶a̶d̶d̶ ̶a̶ ̶s̶e̶p̶a̶r̶a̶t̶e̶,̶ ̶m̶o̶r̶e̶ ̶e̶f̶f̶i̶c̶i̶e̶n̶t̶,̶ ̶w̶a̶t̶e̶r̶ ̶h̶e̶a̶t̶e̶r̶ ̶a̶n̶d̶ ̶r̶u̶n̶ ̶t̶h̶e̶ ̶p̶u̶m̶p̶s̶ ̶l̶e̶s̶s̶ ̶o̶f̶t̶e̶n̶,̶ ̶b̶u̶t̶ ̶i̶t̶'̶s̶ ̶n̶o̶t̶ ̶a̶s̶ ̶s̶i̶m̶p̶l̶e̶ ̶a̶s̶ ̶r̶e̶p̶l̶a̶c̶i̶n̶g̶ ̶o̶n̶e̶ ̶k̶i̶n̶d̶ ̶o̶f̶ ̶h̶e̶a̶t̶e̶r̶ ̶w̶i̶t̶h̶ ̶a̶n̶o̶t̶h̶e̶r̶.̶
Never mind. Apparently I was misinformed. The circulation pump moves the water through the heater, but isn't the primary heat source.