I am a Certified Avian Specialist - AFA Certified Aviculturist - AACC Member in good standing and have over 30 years experience in keeping and raising parrots of all kinds.
Seeds are indeed healthy for most common species of household parrots. Pellets are a mass-media hoax and are not the panacea they would like you to believe. These companies generate a great deal of money - they do not have your pets' best interest at heart.
Seeds are only unhealthy if that is "" all "" you feed your pet.
Parrots ADORE real food! Apples, broccoli, cauliflower, cooked brown rice, grain bread, healthy cereals, corn, peas, carrots, cooked egg, cooked chicken ......etc. etc.. almost any food that is healthy for you is healthy for your bird.
Cockatoos prefer grain based foods such as grain bread, healthy cereal, cooked pasta, corn etc. Most love a bit of organic peanut butter on grain bread cubes!
Cockatiels prefer greens like broccoli and dark green leafy veggies. They aren't big on fruit but many will eat it. Cockatiels and lovebirds are, by nature, seed eaters in the wild.
Most parrots will not eat pellets unless actually forced to do so - some species can suffer serious and life threatening concerns from a pellet diet.
Real, Natural Food is best...... organic is even better. Seeds and nuts are real foods too!
Don't be fooled by the brainwashing!!
Use your head...... :)