What effects besides "mass defect" cause the alpha ladder beyond iron-56/nickel-56 to be endothermic?
There are plenty of misleading statements in Wikipedia and elsewhere on the internet about nucleosynthesis (I am busy searching to see if I have said something similar in the past!)
The reason that the alpha chain does not proceed significantly beyond $^{56}$Ni is that in order to overcome the Coulomb barrier the temperatures need to be so high that the iron-peak nuclei are disintegrated by photons at this temperatures.
I suppose the sense in which the endothermic statement is true is when considering a core made of nickel. In order to produce alpha particles you need to disintegrate some Ni nuclei. This process is highly endothermic and can't be balanced by subsequent fusion.
e.g (And this is a bit simplistic) Photodisintegration of a Ni nucleus into 14 alpha particles requires 88.62 MeV. Then 14 fusion reactions with Ni nuclei, producing Zinc, would give back just 37.9 MeV. In contrast disintegrating $^{52}$Fe into 13 alpha particles needs 80.5 MeV, but 13 fusion reactions of $^{52}$Fe to Ni yields $8.1\times 13= 105.3$ MeV.