Yeah, and it wasn't obvious when I was first looking at the manual diagram - I was like "Huh? It can't be there, that's where the SATA cable connects, it wouldn't fit", before realising it was just not well labelled.
Guess it's because Mini-ITX has limited space, and putting it next to the SATA but on the reverse is probably the most efficient layout - a quick search shows other Mini-ITX boards also have it on the back.
Had the connector been more conveniently accessible I probably would have gone for m.2, but SSD will be good enough, and (once I've got it setup) I can remove the 3.5" HDD and will have absolutely no moving parts. Hooray! :D