Interesting. I have done it both ways, but was converted to sphagnum after much better success and faster growth with more sensitive plants. Haven't gone back to soil or abg mix as a starter medium, but who knows, I may give it a shot if this cutting doesn't make it and I order another umbellata.Probably not a helpful comment at this point, but I tend not to start cuttings in plain sphagnum. I've used regular old 'potting soil' (Fafard, though I bet most are the same) with good results, and also a small pot lined with sphagnum with a well of ABG in the center. I always bury the cut end, and of course keep humidity near 100%.
I've also rooted most vining plants in water, though I've not tried this with Marcgravia.
I have also started many of my vines in the water method. If I get a sufficient enough amount of marcgravia, I'll do an experiment and report back.