A high number of decays per second per gram yes (it's a light element with short half-life) so a small amount of mass gives a lot of activity. The total activity of tritium in the stored water is a few hundred TBq (i.e. a few grams of tritium in all those thousands of tonnes of water).

GP was probably referring to the low energy of the decay (18 keV per decay) which is much lower than other common radionuclides found in waste. For example, Cs-137 releases 600 keV per decay, and Sr-90 releases 576 keV followed by 2274 keV. Tritium is a much lower energy decay, per atom.