That is not what I said. Read it again.
Google some Miniature Horse pedigrees and see if you can find some which list the breeds. You will see some say 'Miniature' and under other horses, it might say 'Shetland'. These Shetlands were mostly smaller ones of the breed, who fit into a Miniature Horse breeding programme.
These days in the US, most Shetlands have taken on a very different look, being much more refined and often taller, than those years ago. Some do still cross Minis and Shetlands but I imagine they only would choose a very small type Shetland these days, if they want the offspring to fall into the Mini category.
Think of Minis as being more of a height breed. With that in mind, a 40"
Shetland would likely not be used for breeding Minis and not registered as one.