Yeah it was only meant to be the 737 700, A319 and 757 that were supposed to be able to operate relatively "normally" on 3-5 hour sectors from St Helena. The article quotes a 138 seat aircraft being used but they don't have a 138 seater... What's the takeoff performance like of a 733? They are configured with 118 seats.
I'd bet they'll use a 733. 738 is likely too big/too marginal and same goes for the 734.