I’m not sure if this is legit, but it’s an interesting concept. Seedlings grafted onto mature stems.
I experimented with this with an ornamental epicactus cross I did last year (Aporocactus x Aporophyllum) and grafted the seedlings a bit further along. My grafts didn't take. Maybe if I used a humidity chamber or used Vaseline to keep in the moisture, but this year I'm just going to wait longer. I think I did five. For most I cut just above the cotyledons, but I know I did at least one with cotyledons attached as shown, and one where I grafted the roots on, since some people said that had worked for them. None of these took.
I've probably done two dozen attempts at grafting epicacti. In that time I think I've had two real successes, which were both on S. grandiflorus with all the areoles removed from the stock. When I haven't removed the areoles, it would just keep pushing out of the base. I've done probably a half dozen grafts onto Hylocereus, never had any success with that. Some of that's technique (or lack thereof).
I think that picture is fake. Even if you managed to graft a seedling with only cotyledons, those don't look like DF cotyledons to me, the seedling's too big, and the callus looks odd. Here's what a S. megalanthus seedling looks like next to a mature (undatus) cutting:
You'll note the woody vascular bundle. Because of that one generally grafts to young growth. I think the main way to graft to H. undatus, like they do for the chlorophyll-lacking cacti, is to cut off a 4-6" new shoot of undatus, graft onto that, root, and once it's got roots on it cut away the areoles. Doing this though means you can't graft onto a huge piece. I've tried doing this without cutting the young undatus piece off, and cutting a few feet of mature growth with it, but the grafts didn't take at all. The pictured piece is from one of those experiments, which is why I have it lying around.
