The article doesn't do a good job of comparing the levels in common terms. Once you do that the situation isn't bad at all. If you have kidney problems your doctor should be going through the dietary changes involved.
From the referenced website:
"Boiled spinach and cocoa powder contain 780 mg/100 g"
"Wilson et al. (9) quantified the oxalic acid in carambola using the HPLC technique. Levels ranging from 0.8 mg/g to 7.3 mg/g were reported."
to change 0.8 mg/g to mg/100g multipley by 100
.8 mg/g x 100= 80mg/100g
7.3 mg/g x 100=703 mg/100g
So, if boiled spinach contains 780mg/100g and Carambola contains, at maximum, 703 mg/100g oxalic acid
then
it can therefore be said that boiled spinach contains MORE oxalic acid than eve SOUR type Carambola, and significantly more than ordinary sweet and especially ripe Carambola.
Many food contain oxylates, another name for oxalic acid, beets, chard(silverbeet), even cinnamon and especially turmeric. Normal people with a varied diet and healthy gut should have no problem.