The question may appear ridiculous at first, as everyone knows Apple have their own Xcode IDE that presumably is used for the development of the OS and apps for iPhone, iPad and the Apple Watch. A deeper analysis of the Apple Watch internals do however show some interesting facts, making the question a lot more sensible.
The new 26 x 28mm S1 system-in-a-package (APL0778) is the heart of the Apple Watch, and it contains no less than 30 individual components. The processor is manufactured in Samsung’s 28nm low-power process, and the 5.2 x 6.2mm CPU is by far the largest component in the package. But wait, there is an STM32 microcontroller in the Apple Watch too!
At least according to Chipworks who disassembled the S1 and looked into its internals. NXP apparently got to sell the NFC controller and secure element, and STMicroelectronics the six-axis gyroscope and accelerometer, as well as the encoder for the digital crown. Texas Instruments got the battery management.
Back to the STM32. What does it do on the Apple Watch? While I have no actual knowledge on the matter, I guess it performs the same tasks as the Apple M7 motion co-processor does on the iPhone 5s and later models. With the iPhone 5s, Apple added a Cortex-M3 processor (rumor says it is a LPC1800 from NXP) to monitor health activities.
As things like step counters are always-on, it makes sense to do that using a low-power Cortex-M processor rather than using the main high-performance (and battery hungry) Cortex-A processor. So my take is the STM32 processor is used for health monitoring on the Apple Watch, just like the NXP LPC1800 processor does on the iPhone.
It is not known what application processor is inside the Apple Watch (Apple license the instruction set architecture from ARM, but otherwise implement the processor design from scratch, as opposed to most ARM chip vendors who license the ready-made implementation too). The main processor in the S1 SiP is most likely a fairly high-powered Cortex-A class processor in any case. One would wonder how much better battery life the Apple Watch would get if it was using the Cortex-M7 processor core instead? That is an interesting thought!
I have no reason to believe TrueSTUDO was in fact used to develop the Apple Watch, but the question may be less ridiculous than originally thought. Atollic TrueSTUDIO support both the STM32 as well as the Cortex-A instruction set, and so it is technically possible. For a tool guy like me, it would be kind of cool if it actually was the case.