You certainly do reach moderate intensity through many of the activities in gardening, sweating and raising your heart rate, for example. Digging, turning over a compost bin, shovelling, barrowing heavy loads up hill would all be examples of this. The other advantage is that you reach lots of different muscle groups. Chopping and hauling logs is also very hard work.
If you are talking about a little gentle weeding then clearly that is not going to give you enough exercise. Yes, heavy gardening can be tough on ageing bodies, but I think pounding a treadmill is just as tough and you don't have anything tangible at the end.
As I said on another thread, many of my neighbours can still do a day's work in the fields in their eighties and they have certainly never been near a gym.