You just need to use your brain,and don't follow blindly everything you read some supposed "authority" says. Internet is full of misleading information from all kinds of authrorities. He probably meant grafted, even if he meant seedling it's probably some anecdotal evidence he heard from somebody. I have 7 mesta trees, 6 years from seed. They grow exactly at the same speed like regular mangosteen and look exactly like regular mangosteen. Until it fruits no one will ever guess the difference.