Yup. I assume there is plenty of value to be found selling donor boards and other parts of the iPhone. Not saying it's a repair shop's fault if they end up with motherboards of a stolen phone, but if I were a thief dealing with an activation-locked phone, my first idea would be a jailbreak, and if that fails, then disassemble the phone and sell the parts.
Presumably thieves already do these things (and more sophisticated things) to extract value out of stolen iPhones.
You really have to be careful with third-party iPhone repairs, especially in China. I had my phone repaired once, and when I got it back...it kind of worked, but they swapped out another part for something that was broken to ensure repeat business. That was in the iPhone 1 days though.
Yes, I'm old. This was in 2009 I think (I bought it in 2007). The e-markets are a lot different now (you can still find obscure shady offices repairing older hardware), and iPhone repairers that were scamming people are probably long gone.
Presumably thieves already do these things (and more sophisticated things) to extract value out of stolen iPhones.