That is a pretty well acknowledged fact :-) Rain water is free of any of the salts, minerals, treatment chemicals, and pharmaceuticals that are found in municipal (tap) water, groundwater, and surface water. It also tends to be slightly acidic as well, which most plants - including JM's - prefer.

Rainwater
also contains nitrate – the most bio-available form of nitrogen. Nitrogen is
one of the three key macro-nutrients that plants need to
thrive–necessary for the development of lush foliage. Many forms of
nitrogen are not actually absorbable by plants. Nitrates, which are made
up of nitrogen and oxygen, are formulated by nature for maximum uptake
by your plants. Plants typically absorb most of their nitrates from the
soil. And one of the best sources of nitrates is natural rainfall!

btw, there is very little difference in just allowing the plants to receive/benefit from natural rainfall as it is to water with collected rainfall, like from rain barrels, cisterns or other collection vessels. It's all good!!