I have an old MacPac waterproof sleeping bag. In summer, I'll happily sleep out in that, as long as the temperature isn't forecast to go below about 5 degrees. It weighs 440g. I try to sleep somewhere a bit soft or grab some stuff to put underneath me. If it is slightly colder, I'd take a sleeping mat.
There is no way I'd get that light with a tent – there's basically no penalty over just sleeping rough in a sleeping bag, and you stay nice and dry if it rains.
The riding is so much nicer when all your sleeping kit weighs <500g, whereas I bet no-one with a tent can get it much under 2kg.
Joe