That post is 6 years old and is recommending a liquid supplement -which is never the best kind. Vitamins degrade very rapidly in water so the best supplements are powder ones that are added to their soft food BUT you are not supposed to supplement anything if you are feeding pellets because they already have way too much of certain vitamins [like Vit A -which is not healthy for birds to consume in its final form and needs to be supplemented through its precursor, betacarotene- or vit D3 which has been found to make the bird lose the ability to move calcium in and out of their bones when given every day].
I don't feed pellets because my research of over 25 years on parrots dietary ecologies have shown that they are not and never will be the best dietary option for a parrot. I feed gloop and raw produce for breakfast and a measured amount of an adequate type of seed and nut mix for dinner so I only give my birds a multivitamin twice a week using a daily dosage each time [so, in reality, I give them 2 or 3 days worth of vitamins every week]. I do this because my diet provides them with everything they need but D3. I use ABBA 2000 which can be used both on soft food [for the parrots[ AND in water [passerines].