Perhaps where that gardener lives did have enough rainfall overall for plants to be able to manage without intervention, but this may not be true for other areas, even areas relatively close by.
In general, mature, woody plants such as large shrubs and trees which have been in situ for some years will be able to cope quite well during periods of drought, though the effects may mean early autumning or leaf loss and reduced growth. Drought conditions experienced by trees can be detected after felling by the narrowness of some of the growth rings, for which one possible cause is a dry year; any fruiting tree or plant may not produce a useable crop in a very dry year in order to conserve water to stay alive. Medium sized woody plants will also cope well for about 3 months, if they've been in situ for some years, but the problem comes with smaller plants such as perennials. Basically, the bigger the root system, the more able temperate zone plants are to cope with relatively short periods of drought, but those which have not been planted long, or are only small plants anyway, may die completely during a 3 month period of drought. Certain plants will always cope better with drought conditions - succulents, mediterranean herbs and shrubs tend to cope better, along with plants such as Yucca and Phormium.
In regard to plants getting used to having too much water, he may have been referring to the common habit people have of walking round with a hosepipe with a sprayer on the end, giving a light dousing to plants in the ground (especially new plantings) daily in the belief this is watering them. Whilst this might be just about okay for temporary plants such as summer bedding, doing this with new long term planting just means the plant doesn't bother to put down deeper roots, since it's learning that you'll be along in a minute to give it the water it needs, so it might as well keep its roots near the surface of the soil because that's where the water is. This is something I've spent years discouraging clients from doing - it's better to give new planting a thorough soak weekly (in the absence of decent rainfall) than a light spray daily. And a pointless exercise watering in this way with mature, established woody plants - if you're going to water those, then a good soak for a couple of hours or more every month to six weeks is best.