They're right. You cannot create a hybrid plant such as that without genetic engineering. Even then, you're just splicing genes for specific traits into the genome, and usually in a very haphazard way given our current level of technology.
But you CAN produce hybrid plants using two plants from the same genus. Generally, the taxanomic classification of the plants has to be very similar in order for it to work, let alone produce any viable offspring. Besides incorporating traits (which are unpredictable) from the two parents, the plant will also exhibit "hybrid vigour", which means that it will be especially resistant to disease, and will be quite strong. As far as cannabis goes, there are very few plants with which this might be possible. The only two plants which come to mind are:
Nepeta, popularly called "Catnip". The plant is mildly psychadelic, even for humans. Our little feline friends, as we all know though, are particularly effected by it.
Humulus, popularly called "Hop". This plant is also mildly psychadelic, producing mostly a sedative effect. And yes, these are the same hopps that are used in brewing beer in the modern world.
Even when producing a hybrid, it's nearly impossible to produce a specimen that is capable of propogation. These can be seen in the example of the mule, or even the liger. Nearly all of them are completely sterile. On the rarest of occasions though, one will be capable of reproduction. These specimens aren't usually selected for breeding, though, because they usually have a high infant mortality rate. The same is true for plants.
Out of the two species I named, hopps is the species that is probably more suited for successful hybridisation with cannabis, since they are in the same family. In fact, it's probably been done before, if even on accident in nature. It does happen. The peyote we have today, many think, is actually a hybrid with exceptional vigour, the original ancestor which passed on the mescaline production genes being lost to history. But this is extremely difficult to do without a background in botany, and you'd probably have more success with interspecies grafting. Though I'm not sure how successful this would be, as the cannabis donor seems to be particularly resistant to taking on the genetic characteristics of stock even within its own genera. I'm thinking this might be because of the way genes are expressed within the species though, and there would be more success with interspecies grafting.
I'm really glad you brought this up. TyPo. I've long been planning on attempting both hybridisation and grafting in the interest of study, personal curiousity, and future ethnobotany. Seriously. I WILL be attempting both at some point in the not so distant future (the latter sooner than the former), and I WILL post the results on here for everyone to see. If an experienced breeder on this forum wants to help me with this, feel free to PM me. I'd appreciate it; we're all here to learn and share our knowledge. I know that's why I'm here. I just wish I wasn't so long-winded. Haha.
~Ethno
Oh! It just occured to me that Leonotis (especially gen. nepetifolia), or Wild Dagga, might be a good candidate since it's closely related. Once again though, not in the same family.