Not sure if serious
Using ones weight to force a manufacturing company to quit producing a competitor's product or risk losing theirs is bullying
As stated by others, we don't have all the details, so we can only speculate based on what was reported. Yes, it can be seen as bullying, but one of the terms in the agreement between Apple and Pegatron may be that they don't produce similar devices for competing companies. That's entirely reasonable, and Apple may just be reinforcing this term.
Companies (like Apple) invest huge amounts of money to help their manufacturing produce their products to their exact specification. We know that Apple pushes the boundaries of what's been done in manufacturing before. They invent new procedures and process, and then train and equip their partners to utilize those procedures and and processes. If what is learned by Pegatron from Apple is then applied to a competitors products, that would seriously piss off Apple, I think.