It appears to me, based on the calculations that they have cited as examples, that the skill level of the crafter has no impact or influence on the ability to synthesize a HQ item.

The odds they are talking about are probably going to be similar to the odds of melding materia. 40% odds of materia melding does not seem like an accurate percentage to me.

There's no mention of durability. Did they confuse themselves or skip that part?

"When using Disciple of the Hand equipment and the appropriate abilities, Quality will improve during synthesis by 300 to 600 points."

What does that mean? One cannot synthesize anything without using DoH equipment. What are the appropriate abilities?

Do they mean Careful Synthesis or crafting ability like Masterpiece?

"(Example)
When all items are HQ items
Final Quality will be between 800–1,000 (odds of producing an HQ item range from 50% to 100%)."

Does this mean that if you achieve a Quality of 1000 that there is 100% probability that the item will be HQ?